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{
"id": 185673,
"topic": "gcn.circulars",
"uuid": "b3898cfd-727f-46ff-9114-6a66c3448771",
"title": "EP250304a: REM NIR upper limits",
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"authors": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/39589"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250304a",
"subject": "EP250304a: REM NIR upper limits",
"createdOn": 1741093246102,
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"circularId": 39589,
"submittedHow": "web"
},
"message_text": "R. Brivio, M. Ferro (INAF-OAB), D. B. Malesani (DAWN/NBI and Radboud), P. D’Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:\n\nWe observed the field of EP250304a (Chen et al., GCN 39580) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the J, H, and K bands, started on 2025 March 04 at 01:46:18 UT (i.e. 0.23 hr after the burst), and lasted for about 2 hours.\n\nFrom preliminary analysis, we do not find any NIR counterpart at the position of the reported optical afterglow (Liu et al., GCN 39583; Saccardi et al., GCN 39585) down to the following 3sigma limit:\n\nJ > 17.5 (Vega; calibrated against the 2MASS catalogue),\n\nat a mid-time of 1.46 hours after the trigger.\n",
"published": "2025-03-04T13:01:28.682011Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-03-04T13:01:28.682033Z",
"modified": "2025-03-04T13:01:28.688848Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 185566,
"topic": "gcn.circulars",
"uuid": "8b6923bd-5699-4e86-9db8-752ecd47cc34",
"title": "EP250304a: Swift/UVOT detection of an optical counterpart with a rising light curve",
"submitter": "Sam Shilling at Lancaster University <shilling.sam@gmail.com>",
"authors": "Sam Shilling at Lancaster University <shilling.sam@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/39587"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250304a",
"subject": "EP250304a: Swift/UVOT detection of an optical counterpart with a rising light curve",
"createdOn": 1741090689814,
"submitter": "Sam Shilling at Lancaster University <shilling.sam@gmail.com>",
"circularId": 39587,
"submittedHow": "web"
},
"message_text": "S. P. R. Shilling (Lancaster U.) reports on behalf of the Swift UVOT team:\n \nSwift/UVOT observed the field of EP250304a (Chen et al., GCN 39580) for a total of 2 ks\nin the U-band starting at 02:48:13 UT, 1.25 hours after the detection by Einstein Probe WXT.\n \nAn optical source consistent with the EP-FXT position (Chen et al., GCN 39580) is detected and\nappears to be rising in brightness. The preliminary detection magnitudes reported below\nare calculated using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).\nThe magnitude at the start of the observation is 19.29 +/- 0.09 (1.25 hours post trigger)\nand rises to 18.91 +/- 0.15 (~2.75 hours post trigger).\n \nWe note that an X-ray source at this position was independently observed by the\nSwift/XRT (Page et al., GCN 39584). We also note that, in addition to the Swift/UVOT detection,\nan optical counterpart at this position has been detected by the TRT (Liu et al., GCN 39583)\nbut has not been detected by MASTER (Lipunov et al., GCN 39581). Finally, that the source\nhas a spectroscopic redshift, observed with the VLT/X-shooter, of z=0.2 (Saccardi et al., GCN 39585).",
"published": "2025-03-04T12:18:39.017632Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-03-04T12:18:39.017665Z",
"modified": "2025-03-04T12:18:39.028880Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 185559,
"topic": "gcn.circulars",
"uuid": "5893ea33-ff71-42dd-b039-242827754b7d",
"title": "EP250304a: Swift-XRT detection of a fading X-ray source",
"submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
"authors": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/39584"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250304a",
"subject": "EP250304a: Swift-XRT detection of a fading X-ray source",
"createdOn": 1741079899630,
"submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
"circularId": 39584,
"submittedHow": "email"
},
"message_text": "K.L. Page, P.A. Evans (U.Leicester) and J. DeLaunay (PSU) report on\nbehalf of the Swift XRT Team:\n\nOn 2025 March 04 at 02:59 UT, Swift started observing EP250304a, 5.2 ks\nafter the Einstein Probe trigger (GCN Circ. 39580). A fading X-ray source\nwas identified, at a position of RA, Dec = 208.39408, -42.8046, which is\nequivalent to\n\nRA (J2000): 13h 53m 34.58s\nDec (J2000): -42d 48′ 16.7″\n\nwith an uncertainty of 4.1 arcsec (radius, 90% confidence). This\nis consistent with the FXT position (GCN Circ. 39580), as well as\nthe TRT optical counterpart given in GCN Circ. 39583.\n\nThe mean observed X-ray flux in the first snapshot was (3.5 +/- 0.4) x\n10^-12 erg cm^-2 s^-1 (0.3 - 10 keV). By the time of the second snapshot,\nat 10.3 ks after the trigger, the source was no longer detected, having\nfaded to a 3-sigma upper limit of 2.1 x 10^-12 erg cm^-2 s^-1.\n",
"published": "2025-03-04T09:20:42.020340Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-03-04T09:20:42.020370Z",
"modified": "2025-03-04T09:20:42.030395Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 185558,
"topic": "gcn.circulars",
"uuid": "e0c48380-13f0-4897-b0a5-c12f46a9dd0b",
"title": "EP250304a: TRT optical counterpart detection",
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"authors": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/39583"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250304a",
"subject": "EP250304a: TRT optical counterpart detection",
"createdOn": 1741076269981,
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"circularId": 39583,
"submittedHow": "web"
},
"message_text": "X. Liu, J. An, N.C. Sun (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Q. Jiang, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, Y.N. Wang (NAOC) report on behalf of a large collaboration:\n\nWe observed the field of EP250304a (Chen et al, GCN 39580), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 02:18:49 UTC on 2025-03-04, i.e., ~1.12 hr after the EP/WXT trigger. A series of B-/V-/R-/I- band frames were obtained. \n\nAn uncatalogued and varying optical source is detected within the EP/FXT error circle (Chen et al, GCN 39580) at coordinates\n\nR.A. (J2000) = 13:53:34.68\nDec. (J2000) = -42:48:16.71\n\nwith an uncertainty of ~ 1 arcsec.\n\nPreliminary photometry shows that the source has B = 20.5 +/- 0.1 mag at 1.39 hr post-trigger, and R = 20.8 +/- 0.1 mag at 2.4 hr post-trigger, calibrated with SkyMapper DR2 catalog converted using Lupton (2005) equations and not corrected for Galactic extinction.\n\nWe thus think this source is the optical counterpart of EP250304a.",
"published": "2025-03-04T08:19:03.653071Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-03-04T08:19:03.653105Z",
"modified": "2025-03-04T08:19:03.661307Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 185564,
"topic": "gcn.circulars",
"uuid": "6db678a3-49fe-40a6-aa73-4563de3586bf",
"title": "EP250304a: VLT/X-shooter redshift z = 0.200",
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"authors": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/39585"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250304a",
"subject": "EP250304a: VLT/X-shooter redshift z = 0.200",
"createdOn": 1741082043706,
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"circularId": 39585,
"submittedHow": "web"
},
"message_text": "A. Saccardi (CEA/Irfu), Z. P. Zhu (NAOC), B. Schneider (LAM), D. Xu (NAOC), L. Izzo (INAF/OAC and DARK/NBI), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (U. Leicester), S. D. Vergani (CNRS - Paris Observatory/LUX) report on behalf of the Stargate collaboration:\n\nWe observed the optical counterpart (Liu et al., GCN 39583; see also Page et al., GCN 39584) of the fast X-ray transient EP250304a (Chen et al., GCN 39580) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph.\n\nIn a 30-s r-band image taken with the acquisition camera (mid-time 2025 Mar 4.302 UT, or 5.7 hr after trigger), we detect the optical counterpart with a magnitude r = 20.97 +/- 0.03 (AB), calibrated against nearby stars from the SkyMapper catalog (Wolf et al. 2018, doi:10.4225/41/593620ad5b574).\n\nOur spectra cover the wavelength range 3000-21000 AA, and consist of 4 exposures of 600 s each. The observation mid time was 2025 Mar 4.3248 UT (6.254 hr after the GRB). \n\nIn a preliminary reduction of the spectra, we detect continuum over the entire covered wavelength range. We detect emission lines in the VIS arm, which we identify as Halpha and [O III] 5007 AA at the common redshift of z = 0.200. We also identify the Mg II doublet (2796,2804) in absorption at a consistent redshift.\n\nThe spectrum continues to rise towards the blue end down to the cut-off of the UVB arm (~3100 AA). This spectral shape is qualitatively similar to what was seen in EP250108a / AT2025kg (e.g., Zhu et al., GCN 38908). We encourage further follow-up of this potentially interesting target.\n\nWe acknowledge expert support from the ESO staff in Paranal, in particular Diego Parraguez.\n",
"published": "2025-03-04T09:54:45.591397Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-03-04T09:54:45.591427Z",
"modified": "2025-03-04T09:54:45.608877Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186251,
"topic": "gcn.circulars",
"uuid": "315c6ad7-918e-4230-b478-c69c4ba8fafc",
"title": "EP250321A: J band upper limit by SYSU 80cm telescope",
"submitter": "Yan Yu <yuyan35@mail2.sysu.edu.cn>",
"authors": "Yan Yu <yuyan35@mail2.sysu.edu.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40054"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40054....1Y",
"eventId": "EP250321A",
"subject": "EP250321A: J band upper limit by SYSU 80cm telescope",
"createdOn": 1743780730068,
"submitter": "Yan Yu <yuyan35@mail2.sysu.edu.cn>",
"circularId": 40054,
"submittedHow": "web"
},
"message_text": "Yan Yu, Jin-Ji Li, Jia-Qi Lin, Wei-Sen Huang, Zhong-Nan Dong, Pu Lin, Hao-Nan Yang, Hao-Ran Zhang, Chun Chen, P H Thomas Tam, Rong-Feng Shen, Bin Ma(Sun Yat-sen University) report on behalf of the SYSU 80cm telescope team:\n\nWe observed the field of EP250321A (EP GCN 39800; MASTER-OAFA GCN 39803; Zhu et al., GCNs 39809; Swift-XRT GCN 39811) using the Sun Yat-sen University 80cm infrared telescope with 90 x 20 s exposures in J band. The calculated position is RA. = 179.2625 deg, DEC = 17.3628 deg J2000, from EP observation. Our observations began at 2025-3-22 13:25:00 UTC, 31.25 hours after the EP trigger.\n\nWe do not detect any counterpart at the position of the optical afterglow (Fu et al., GCN 39804; Perez-Garcia et al., GCNs 39805; Brivio et al., GCNs 39807; Becerra et al., GCNs 39810; Pérez-Fournon et al., GCN 39812; Lee et al., GCN 39815; Ghosh et al., GCN 39816; Han et al., GCN 39817; Jin et al., GCN 39822; Zhu et al., GCN 39827; Gill et al., GCN 39831; Zheng et. al, GCN 39832; Pankov et al., GCN 39836; Bochenek et. al, GCN 7939837; Zhu et. al, GCN 39854), down to a 5-sigma depth of J~ 17.6 Vega magnitudes.",
"published": "2025-05-10T16:09:49.401744Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.401762Z",
"modified": "2025-05-10T16:09:49.405038Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186309,
"topic": "gcn.circulars",
"uuid": "5bff7e51-a1d5-41b3-8c44-35cddf5a9ca1",
"title": "EP250402a: Einstein Probe detection of a fast X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40017"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40017....1L",
"eventId": "EP250402a",
"subject": "EP250402a: Einstein Probe detection of a fast X-ray transient",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"editedOn": 1743685776573,
"createdOn": 1743662011832,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40017,
"submittedHow": "web"
},
"message_text": "R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:\n\nWe report on the detection of a fast X-ray transient EP250402a (trigger ID: 01709133860) at 2025-04-02T12:29:06.850 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The position of the source is R.A., DEC. = 172.168, -47.290 deg (J2000) with an uncertainty of 2.5 arcmin (radius, 90% C.L. statistical and systematic). The transient event lasted for about 80 seconds, with a peak 0.5-4 keV flux of 5.7 x 10^-9 erg/s/cm^2. \n\nThe 0.5-4.0 keV spectrum can be fitted with an absorbed powerlaw model, with the column density nH fixed at the Galactic value of 1.43 x 10^21 cm^-2 and a photon index of 1.3 (+0.7/-0.7). The unabsorbed 0.5-4.0 keV flux is 1.2 (+0.6, -0.4) x10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.\n\nFollow-up observations by EP-FXT are prepared.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).\n",
"published": "2025-05-10T16:09:49.894755Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.894773Z",
"modified": "2025-05-10T16:09:49.898156Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186283,
"topic": "gcn.circulars",
"uuid": "d43376cc-966e-4774-9400-7e055d2016d2",
"title": "EP250402a: EP-FXT follow-up observations",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40038"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40038....1L",
"eventId": "EP250402a",
"subject": "EP250402a: EP-FXT follow-up observations",
"createdOn": 1743755941747,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40038,
"submittedHow": "web"
},
"message_text": "R.-Z. Li (YNAO, CAS), H. Q. Cheng (NAO, CAS), Q. C. Zhao, Z. H. Yang (IHEP, CAS), Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:\n\nThe X-ray transient EP250402a was detected by EP-WXT (Li et al., GCN 40017). Two follow-up observations were conducted by EP-FXT.\n\nThe first follow-up observation was performed at 2025-04-03T07:25:06 (UTC), about 18.93 hours after the trigger, with an exposure time of 1 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source at the coordinates (J2000):\nR.A., Dec. = 172.1798, -47.2954 deg,\nwith an uncertainty of about 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). This source lies within the EP-WXT error circle (Li et al., GCN 40017). The 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the column density nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index of 2.9 (+1.2, -1.0). The unabsorbed 0.5-10 keV flux is 1.9 (+1.2, -0.7) e-13 erg/s/cm^2.\n\nThe second follow-up observation was performed at 2025-04-03T17:11:45 (UTC), about 28.71 hours after the trigger, with an exposure time of 4.4 ks. The 0.5-10 keV spectrum can also be fitted with an absorbed power-law model, with nH fixed at the Galactic value of 1.43e21 cm^-2, and a photon index fixed at 2.9. The unabsorbed 0.5-10 keV flux is 9.1 (+5.0, -3.5) e-14 erg/s/cm^2.\n\nAll uncertainties quoted above are at the 90% confidence level.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).\n",
"published": "2025-05-10T16:09:49.675274Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.675291Z",
"modified": "2025-05-10T16:09:49.678396Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186475,
"topic": "gcn.circulars",
"uuid": "a1bf03b0-62f9-4ccc-8437-997d4dea6905",
"title": "EP250402a: Optical upper limit with Kinder observations",
"submitter": "Janet Chen at National Central University <janetstars@gmail.com>",
"authors": "Janet Chen at National Central University <janetstars@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40046"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40046....1A",
"eventId": "EP250402a",
"subject": "EP250402a: Optical upper limit with Kinder observations",
"createdOn": 1743768083056,
"submitter": "Janet Chen at National Central University <janetstars@gmail.com>",
"circularId": 40046,
"submittedHow": "web"
},
"message_text": "A. Aryan, Y.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Brennan (OKC), A. K. H. Kong (NTHU), J. Gillanders (Oxford), S. J. Smartt (Oxford/QUB), Y. J. Yang, A. Sankar. K, Y.-C. Pan, C.-C. Ngeow, M.-H. Lee, H.-Y. Hsiao, C.-S. Lin, H.-C. Lin, C.-H. Lai, J.-K. Guo (all NCU), S. Yang, L. L. Fan, Z. N. Wang, G. H. Sun (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:\n\nWe observed the field of the fast X-ray transient EP250402a (Li et al., GCN 40017, GCN 40038) using the 40cm SLT telescope at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al. 2024, arXiv:2406.09270). The first SLT epoch of observations in the r-band started at 14:18 UT on the 3rd of April 2025 (MJD = 60768.538), ~24.43 hrs after the EP trigger.\n\nWe utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. We did not detect any uncataloged optical counterpart candidate within the EP-WXT/EP-FXT localization error circle of radius 2.5 arcminutes/10 arcseconds. The field of FXT is only covered by SkyMapper. \n\nWe further employed the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, 62) to perform PSF photometry on our stacked frames. The details of the observations and measured three sigma upper limits (in the AB system) were as follows: \n\nTelescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass \nSLT | r | 60768.538 | 24.43 | 300 * 24 | >20.5 | 1\".83 | 3.11 \n\nThe presented upper limit was calibrated using the field stars from the SkyMapper catalog and was not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.38 mag, respectively, in the direction of the transient (Schlafly & Finkbeiner 2011).\n\n",
"published": "2025-05-10T16:09:51.503586Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.503603Z",
"modified": "2025-05-10T16:09:51.507002Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186258,
"topic": "gcn.circulars",
"uuid": "9ac16454-47e2-4c7d-bd49-388abcb421a7",
"title": "EP250402a: Optical upper limit with PD 1.3m observations",
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"authors": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40081"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40081....1Q",
"eventId": "EP250402a",
"subject": "EP250402a: Optical upper limit with PD 1.3m observations",
"createdOn": 1743872725155,
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"circularId": 40081,
"submittedHow": "web"
},
"message_text": "Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz Bauer (PUC), Antonio Martin-Carrillo (UCD), Andrew J. Levan (Radboud), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud) report on behalf of a larger collaboration:\n\nWe obtained photometric observations of the field of the fast X-ray transient EP250402a discovered by Einstein Probe (Li et al., GCN 40017, GCN 40038) using the Planaetry Defense 1.3m telescope localized in Cerro Tololo Inter-American Observatory, Chile. We obtained 30 images of 120 s each in the r-band filter beginning at 2025-04-03 07:06:41 UT and with a middle time at T0+0.797 days after the EP trigger.\n\nWithin the EP-FXT error circle, we detect no plausible optical counterpart down to a 3sigma limit of r > 21.8 AB mag, calibrated against SkyMapper nearby reference stars\n\nWe acknowledge the excellent support from the PD 1.3m staff, in particular Tyler Linder.\n",
"published": "2025-05-10T16:09:49.461854Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.461873Z",
"modified": "2025-05-10T16:09:49.465138Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186288,
"topic": "gcn.circulars",
"uuid": "66f47c0d-5b11-4902-bf1a-8f97a3b11044",
"title": "EP250404a: AKO Optical Afterglow Detection",
"submitter": "Mohammad Odeh at Al Khatim Observatory M44 <mshodeh@gmail.com>",
"authors": "Mohammad Odeh at Al Khatim Observatory M44 <mshodeh@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40064"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40064....1O",
"eventId": "EP250404a",
"subject": "EP250404a: AKO Optical Afterglow Detection",
"createdOn": 1743791728306,
"submitter": "Mohammad Odeh at Al Khatim Observatory M44 <mshodeh@gmail.com>",
"circularId": 40064,
"submittedHow": "email"
},
"message_text": "Mohammad Odeh (Al-Khatim Observatory, AKO, operated by the International\nAstronomical Center in Abu Dhabi, UAE), and Shaikha Alshamsi and Nidhal\nGuessoum (American University of Sharjah, UAE), report:\n\nWe observed the field of EP250404a detected by EP (Hu et al., GCN 40051),\nusing our 0.36m f/7.7 robotic telescope. The observation session began on 04\nApril 2025 at 15:47 UT and continued until 16:56 UT, with a midpoint at\n16:22 UT, approximately 2 hours after the trigger.\n\n \n\nWe obtained multiple 180-second exposures using the Ic filter. The optical\nafterglow was clearly detected at:\n\nR.A. (J2000): 08:20:14.59 \n\nDec. (J2000): +35:31:42.3\n\n \n\nOur detection is consistent with the results of (Jiang et al., GCN 40052;\nPerez-Garcia et al., GCN 40053; Du et al., GCN 40058; Konno et al., GCN\n40063).\n\n \n\nThe following observation was calculated using the Atlas catalogue as a\nreference:\n\n----------------------------------------------------------------------------\n-\n\nObsTime (mid), Exposure (sec), Filter, Mag\n\n----------------------------------------------------------------------------\n-\n\n2025-04-04T16:21:38Z, 20 x 180s (stacked), Ic, 17.5 +/- 0.35\n\n----------------------------------------------------------------------------\n-\n\nThe magnitude is not corrected for galactic extinction.\n\n",
"published": "2025-05-10T16:09:49.716016Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.716034Z",
"modified": "2025-05-10T16:09:49.719202Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186350,
"topic": "gcn.circulars",
"uuid": "2fbc8dc7-eff1-47ad-b044-90def2602f61",
"title": "EP250404a: BOOTES-4/JGT optical afterglow detection",
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"authors": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40053"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40053....1P",
"eventId": "EP250404a",
"subject": "EP250404a: BOOTES-4/JGT optical afterglow detection",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"editedOn": 1743792381613,
"createdOn": 1743780684335,
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"circularId": 40053,
"submittedHow": "web"
},
"message_text": "I. Perez-Garcia, S.-Y. Wu, E. Fernandez-Garcia, M. D. Caballero-Garcia, G. Garcia-Segura, S. Guziy, R. Sanchez-Ramirez and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar (Univ. de Malaga), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXU), and D. R. Xiong, J. M. Bai, Y. F. Fan, C. J. Wang, Y. X. Xin, X. H. Zhao, J. R. Mao, B. K. Lun, K. Ye (Yunnan Observatories/CAS, Kunming) on behalf of a larger collaboration, report:\n\nFollowing the detection of EP250404a by Einstein Probe (Hu et al. GCNC [40051](https://gcn.nasa.gov/circulars/40051)) the 0.6m BOOTES-4/MET robotic telescope at Lijiang Astronomical Observatory (China) automatically responded to this X-ray transient starting on 2025-04-04 14:39:39UT (20 min after detection and 15 after notification). Series of images in clear filter were gathered and we detect and uncatalogud source constistent with position reported by Jiang et al. (GCN [40052](https://gcn.nasa.gov/circulars/40052)) Using GaiaDR3 Gmag as reference we measure an initial magnitude of 14.8 +/- 0.03. Within 20 minutes, the source faded to magnitude 16.1. Further analysis of the additional images is ongoing.\n\nWe thank the staff at Lijiang observatory for their excellent support.",
"published": "2025-05-10T16:09:50.295832Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.295851Z",
"modified": "2025-05-10T16:09:50.298893Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186479,
"topic": "gcn.circulars",
"uuid": "f38ac636-18e1-425f-a5c8-291264414ad2",
"title": "EP250404A: COLIBRÍ/DDRAGO Optical Observations",
"submitter": "Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com>",
"authors": "Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40073"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40073....1A",
"eventId": "EP250404A",
"subject": "EP250404A: COLIBRÍ/DDRAGO Optical Observations",
"createdOn": 1743841894944,
"submitter": "Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com>",
"circularId": 40073,
"submittedHow": "web"
},
"message_text": "Ny Avo Rakotondrainibe (LAM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (U Roma), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Sarah Antier (OCA/IJCLAB), Stéphane Basa (UAR Pytheas), Diego López-Cámara (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Ramandeep Gill (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Antonio de Ugarte Postigo (LAM) and Benjamin Schneider (LAM) report:\n\nWe imaged the field of EP250404A (Hu et al., GCN Circ. 40051) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-05 UTC.\n\nWe observed from 2025-04-05 05:39 to 06:18 UTC (T+15.3 to T+16.0 hours after the trigger) and obtained 1920 seconds of exposure in total in the r filter. Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nWe detected the optical counterpart (Jiang et al., GCN Circ. 40052, Perez-Garcia et al., GCN Circ. 40053, Du et al., GCN Circ. 40058, Zhu et al., GCN Circ. 40061, Konno et al. GCN Circ. 40063, Odeh et al., GCN Circ. 40064, Lipunov et al., GCN Circ. 40066, He et al., GCN Circ. 40069, Perez-Fournon et al., GCN Circ. 40071, Jelinek et al., GCN Circ. 40072) with an AB magnitude of:\n\nr = 21.99 +/- 0.15. \n\nCompared with the values previously reported, we estimate the fading nature of the OT with a temporal index ~1.4 after T+1.7 hours.\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.",
"published": "2025-05-10T16:09:51.539279Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.539297Z",
"modified": "2025-05-10T16:09:51.542415Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186285,
"topic": "gcn.circulars",
"uuid": "9ff10605-a133-4b89-9f21-8b50a8974979",
"title": "EP250404a: Einstein Probe detection of an X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40051"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40051....1H",
"eventId": "EP250404a",
"subject": "EP250404a: Einstein Probe detection of an X-ray transient",
"createdOn": 1743779029367,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40051,
"submittedHow": "web"
},
"message_text": "J. W. Hu (NAO, CAS), Q. C. Liu (THU), B. B. Zhang (BNU), Y. L. Hua (PMO, CAS), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250404a. The transient triggered EP-WXT (ID: 01709133918) at 2025-04-04T14:19:59 (UTC). The WXT position of the source is R.A. = 125.064 deg, DEC = 35.511 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 125.0575 deg, DEC = 35.5305 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). It is noted that EP250404a is temporally and spatially coincident with GRB 250404A (Fermi team, GCN Circ. 40050).\n\nFurther information will be updated when the telemetry data is received.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:49.691486Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.691504Z",
"modified": "2025-05-10T16:09:49.694528Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186476,
"topic": "gcn.circulars",
"uuid": "6979a7e6-429d-4584-8d75-4db13666229e",
"title": "EP250404a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40066"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40066....1L",
"eventId": "EP250404a",
"subject": "EP250404a: Global MASTER-Net observations report",
"createdOn": 1743794859645,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40066,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP250404a ( EP Team et al., GCN 40051) errorbox 13675 sec after notice time and 16626 sec after trigger time at 2025-04-04 18:57:05 UT, with upper limit up to 19.5 mag. The observations began at zenith distance = 66 deg. The sun altitude is -28.6 deg. \n\nThe galactic latitude b = 33 deg., longitude l = 187 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2832935\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 16657 | 2025-04-04 18:57:05 | MASTER-Tunka | (08h 19m 51.88s , +36d 00m 34.0s) | C | 60 | 18.6 | \n 16717 | 2025-04-04 18:57:05 | MASTER-Tunka | (08h 19m 51.88s , +36d 00m 34.0s) | C | 180 | 19.3 | Coadd \n 16728 | 2025-04-04 18:58:16 | MASTER-Tunka | (08h 19m 58.07s , +36d 00m 34.7s) | C | 60 | 18.6 | \n 16800 | 2025-04-04 18:59:28 | MASTER-Tunka | (08h 19m 55.11s , +36d 02m 04.0s) | C | 60 | 18.6 | \n 16872 | 2025-04-04 19:00:41 | MASTER-Tunka | (08h 19m 55.07s , +36d 00m 32.5s) | C | 60 | 18.7 | \n 16932 | 2025-04-04 19:00:41 | MASTER-Tunka | (08h 19m 55.07s , +36d 00m 32.5s) | C | 180 | 19.4 | Coadd \n 16950 | 2025-04-04 19:01:58 | MASTER-Tunka | (08h 19m 57.66s , +36d 02m 04.7s) | C | 60 | 18.6 | \n 17021 | 2025-04-04 19:03:09 | MASTER-Tunka | (08h 19m 52.37s , +36d 01m 15.7s) | C | 60 | 18.7 | \n 17095 | 2025-04-04 19:04:24 | MASTER-Tunka | (08h 19m 51.30s , +36d 02m 12.7s) | C | 60 | 18.7 | \n 17155 | 2025-04-04 19:04:24 | MASTER-Tunka | (08h 19m 51.30s , +36d 02m 12.8s) | C | 180 | 19.5 | Coadd \n 17381 | 2025-04-04 19:09:09 | MASTER-Tunka | (08h 19m 52.28s , +35d 59m 59.8s) | C | 60 | 18.7 | \n 17454 | 2025-04-04 19:10:22 | MASTER-Tunka | (08h 19m 58.20s , +36d 00m 10.2s) | C | 60 | 18.7 | \n 17527 | 2025-04-04 19:11:35 | MASTER-Tunka | (08h 19m 54.28s , +36d 01m 24.3s) | C | 60 | 18.7 | \n 17587 | 2025-04-04 19:11:35 | MASTER-Tunka | (08h 19m 54.28s , +36d 01m 24.2s) | C | 180 | 19.4 | Coadd \n 17599 | 2025-04-04 19:12:48 | MASTER-Tunka | (08h 19m 54.27s , +35d 59m 56.4s) | C | 60 | 18.6 | \n 17670 | 2025-04-04 19:13:59 | MASTER-Tunka | (08h 19m 57.20s , +36d 01m 34.1s) | C | 60 | 18.6 | \n 17743 | 2025-04-04 19:15:12 | MASTER-Tunka | (08h 19m 51.23s , +36d 00m 41.3s) | C | 60 | 18.5 | \n 17803 | 2025-04-04 19:15:12 | MASTER-Tunka | (08h 19m 51.23s , +36d 00m 41.3s) | C | 180 | 19.3 | Coadd \n 17816 | 2025-04-04 19:16:24 | MASTER-Tunka | (08h 19m 51.76s , +36d 01m 37.4s) | C | 60 | 18.6 | \n 17890 | 2025-04-04 19:17:39 | MASTER-Tunka | (08h 19m 57.64s , +36d 00m 34.5s) | C | 60 | 18.7 | \n 17962 | 2025-04-04 19:18:51 | MASTER-Tunka | (08h 19m 51.82s , +35d 59m 30.3s) | C | 60 | 18.6 | \n 18035 | 2025-04-04 19:20:04 | MASTER-Tunka | (08h 19m 57.26s , +35d 59m 42.1s) | C | 60 | 18.6 | \n 18108 | 2025-04-04 19:21:16 | MASTER-Tunka | (08h 19m 53.77s , +36d 01m 07.0s) | C | 60 | 18.5 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:51.511937Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.511955Z",
"modified": "2025-05-10T16:09:51.515108Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186355,
"topic": "gcn.circulars",
"uuid": "9f41544a-96f1-47a8-87e6-a86d19b2e0c5",
"title": "EP250404a / GRB 250404A: J band observations by SYSU 80cm telescope",
"submitter": "Chun Chen at Sun Yat-sen University, SYSU 80cm telescope team <chench386@mail2.sysu.edu.cn>",
"authors": "Chun Chen at Sun Yat-sen University, SYSU 80cm telescope team <chench386@mail2.sysu.edu.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40088"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40088....1C",
"eventId": "EP250404a",
"subject": "EP250404a / GRB 250404A: J band observations by SYSU 80cm telescope",
"createdOn": 1743942384201,
"submitter": "Chun Chen at Sun Yat-sen University, SYSU 80cm telescope team <chench386@mail2.sysu.edu.cn>",
"circularId": 40088,
"submittedHow": "web"
},
"message_text": "Chun Chen, Xia Li, Zhong-Nan Dong, Jia-Qi Lin, Wei-Sen Huang, Jin-Ji Li, Pu Lin, Hao-Nan Yang, Yan Yu, Hao-Ran Zhang, Hao-Yuan Zheng, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm telescope team:\n\nWe observed the field of EP250404a detected by Einstein Probe (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB250404A (Fermi GBM team, GCN40050; Fermi GBM team, GCN40067), using the Sun Yat-sen University 80cm infrared telescope with 183 x 20 s exposures in J band. The calculated position is R.A. = 125.0575 deg, DEC = 35.5305 deg J2000, from EP/FXT observation. Our observations began at 2025-4-4 15:25:20 UTC, 1.09 hours after the EP trigger. \n\nWe detected a counterpart at the position of the optical afterglow (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al. GCN 40063; Odeh et al., GCN 40064; Lipunov et al., GCN 40066; He et al., GCN 40069; Perez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074; Ruocco, GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., GCN 40083; Li et al., GCN 40084) in the stacked images. The preliminary photometric magnitudes in J band are below:\n\nStart Time (UTC) |Band | Exp | Mag (Vega)\n--------------------|-----|-------|------------------\n2025/4/4 15:25:20 | J | 200s | 15.30+/-0.08\n2025/4/4 15:28:40 | J | 200s | 15.28+/-0.09\n2025/4/4 15:32:20 | J | 200s | 15.47+/-0.10\n2025/4/4 15:35:40 | J | 200s | 15.76+/-0.11\n2025/4/4 15:39:00 | J | 360s | 15.83+/-0.10\n2025/4/4 15:50:20 | J | 400s | 16.05+/-0.12\n2025/4/4 15:57:00 | J | 400s | 15.99+/-0.12\n2025/4/4 16:03:40 | J | 400s | 15.99+/-0.12\n2025/4/4 16:10:40 | J | 400s | 16.29+/-0.15\n2025/4/4 16:17:40 | J | 1100s | 16.21+/-0.11\n\nThe photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitudes are not corrected for Galactic extinction.\n",
"published": "2025-05-10T16:09:50.339128Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.339146Z",
"modified": "2025-05-10T16:09:50.342444Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186356,
"topic": "gcn.circulars",
"uuid": "2317c8e4-8445-4b61-bd7b-147f7429250a",
"title": "EP250404a / GRB 250404A: Montarrenti Observatory optical observations ",
"submitter": "Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it>",
"authors": "Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40093"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40093....1L",
"eventId": "EP250404a",
"subject": "EP250404a / GRB 250404A: Montarrenti Observatory optical observations ",
"createdOn": 1743947351776,
"submitter": "Simone Leonini at Montarrenti Observatory (Siena, Italy) <s.leonini@iol.it>",
"circularId": 40093,
"submittedHow": "web"
},
"message_text": "S. Leonini, A. Lorini, M. Conti, P. Rosi, L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy, part of UAI/SSV-GRB section), M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy) and K. Kalinowski (Aarhus University, Department of Physics and Astronomy) report:\n\nwe observed the field of the X-ray transient EP250404a (Einstein Probe mission, Trigger ID 01709133918) probably associated with GRB 250404A (Fermi team, GCN 40050) with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88).\n\nThe observations were started at 2025-04-04 18:36:04 UT (approximately 4 hours after burst) ending at 2025-04-04 22:00:50 UT stacking a set of Rc-band CCD images. Observations were performed under thin cloud cover in the second part of the night. \n\nThe optical afterglow (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al, GCN 40064; He et al., GCN 40069; Perez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al, GCN 40073; Taguchi et al., GCN 40074; Ruocco et al., GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., GCN 40083; Li et al., GCN 40084; Yin et al., GCN 40085; Pankov et al., GCN 40086; Chu Chen et al., GCN 40088 and Chen et al., GCN 40089) was clearly detected at the following position:\n\nRA (J2000.0) 08h 20m 14.57s +/-0.06 \nDecl. (J2000.0) +35° 31' 42.0\" +/-0.04\n\nPreliminary photometry was obtained using nearby PanSTARRS stars as follows: \n\nObservation Mid-Time T-T0 (hr) Exposure Filter Mag. Err. \n2025-04-04 18:36:24 UT 4.27 50x40s Rc 19.60 +/-0.10 \n2025-04-04 19:11:30 UT 4.85 50x40s Rc 19.67 +/-0.10\n2025-04-04 19:46:31 UT 5.43 50x40s Rc 19.81 +/-0.15\n2025-04-04 20:21:33 UT 6.02 54x40s Rc 20.25 +/-0.20\n2025-04-04 21:07:10 UT 6.78 78x40s Rc 20.16 +/-0.12\n\nMagnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied.\n \nAlong the span of our observations and within our uncertainties, the afterglow evolves in agreement with a power-law decay with a rate alpha ~1.40.\n",
"published": "2025-05-10T16:09:50.347969Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.347988Z",
"modified": "2025-05-10T16:09:50.351167Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186352,
"topic": "gcn.circulars",
"uuid": "dae4c635-25d8-4510-bf68-1ab67329b285",
"title": "EP250404A / GRB250404A: Ondrejov D50 optical afterglow detection",
"submitter": "Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>",
"authors": "Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40072"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40072....1J",
"eventId": "GRB 250404A",
"subject": "EP250404A / GRB250404A: Ondrejov D50 optical afterglow detection",
"createdOn": 1743824974733,
"submitter": "Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek@asu.cas.cz>",
"circularId": 40072,
"submittedHow": "email"
},
"message_text": "M. Jelinek, J. Strobl, F. Novotny, R. Hudec (ASU CAS Ondrejov) report:We observed the field of EP250404a detected by EP (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A (Fermi team, GCN 40050; Mukherjee et al., GCN 40067), using the D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. Our observations started at 15:50 UT on 2025-04-04, approximately 5.5 hours after the trigger. We performed a series of 120s exposures in the SDSS i' filter. The optical counterpart, previously reported by several teams (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Konno et al., GCN 40063; Odeh et al., GCN 40064; He et al., GCN 40069; Pérez-Fournon et al., GCN 40071), is detected in our combined image of 25x120s exposures. The PanSTARRS-calibrated magnitude of the source is i' = 19.5 +/- 0.1, not corrected for Galactic extinction.",
"published": "2025-05-10T16:09:50.312651Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.312670Z",
"modified": "2025-05-10T16:09:50.315645Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186474,
"topic": "gcn.circulars",
"uuid": "6634a101-a3f1-4d9a-b9c1-1208973cd6e4",
"title": "EP250404a: LAST detection of optical counterpart",
"submitter": "Ruslan Konno at Weizmann Institute of Science <ruslan.konno@desy.de>",
"authors": "Ruslan Konno at Weizmann Institute of Science <ruslan.konno@desy.de>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40063"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40063....1K",
"eventId": "EP250404a",
"subject": "EP250404a: LAST detection of optical counterpart",
"createdOn": 1743790653639,
"submitter": "Ruslan Konno at Weizmann Institute of Science <ruslan.konno@desy.de>",
"circularId": 40063,
"submittedHow": "web"
},
"message_text": "R. Konno (WIS), S. Garrappa (WIS), E. A. Zimmerman (WIS), A. Horowicz (WIS), E. O. Ofek (WIS), S. Ben-Ami (WIS), D. Polishook (WIS), O. Yaron (WIS), P. Chen (WIS), A. Krassilchtchikov (WIS), Y. M. Shani (WIS), E. Segre (WIS), A. Gal-Yam (WIS), S. Spitzer (WIS), and K. Rybicki (WIS) on behalf of the LAST Collaboration\n\nWe report observations of the X-ray transient EP250404a (Hu et al., GCN 40051), detected by Einstein Probe, with the Large Array Survey Telescope (LAST; Ofek et al. 2023 PASP 135, 5001; Ben-Ami et al. 2023 PASP 135, 5002).\n\nWe observe the field of EP250404a using 4 parallel telescopes, each with a FoV of 7.4 deg^2 and no filter (clear - similar to the GAIA Bp band) over several epochs. In each epoch, we coadd 20 images with each of 20s exposure, and the limiting magnitude of a single coadd is around 20.5 (AB mag). We clearly detect the optical counterpart reported by Jiang et al. (GCN 40052), Perez-Garcia et al. (GCN 40053), and Du et al. (GCN 40058). The earliest detection with a 20x20s exposure image is confirmed at 2025-04-04 17:01:04 UTC (T-T0=2.68h) at an AB magnitude of 19.31 +/- 0.07. LAST is scheduled to continue observations of the source, and additional analysis will follow.\n\nLAST is a survey telescope array of the Weizmann Astrophysical Observatory (https://www.weizmann.ac.il/wao/).\n\n",
"published": "2025-05-10T16:09:51.493285Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.493302Z",
"modified": "2025-05-10T16:09:51.497805Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186415,
"topic": "gcn.circulars",
"uuid": "f3dc7895-ef01-413d-8480-34d1417abffb",
"title": "EP250404a: LCO optical counterpart detection",
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"authors": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40071"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40071....1P",
"eventId": "EP250404a",
"subject": "EP250404a: LCO optical counterpart detection",
"createdOn": 1743814357592,
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"circularId": 40071,
"submittedHow": "web"
},
"message_text": "I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), D. Aguado, A. López-Oramas, D. Nespral (IAC and ULL), N.C. Sun (UCAS), W. Li, Y. Wang, and Z. Niu (NAOC)\n\nWe report on Las Cumbres Observatory Global Telescope network (LCOGT) observations of the X-ray transient EP250404a (Hu et al., GCN circ. 40051), that is temporally and spatially consistent with the likely long GRB 250404A, detected by Fermi/GBM (Fermi GBM team, GCN 40050; and Mukherjee et al., GCN circ. 40067).\n\nWe observed the field of EP250404a with the two LCOGT 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at Teide Observatory (Tenerife, Spain) in the SDSS i, g, and r filters,\nstarting at 2025-04-04 20:27:26 UTC, about 6.12 hours after the EP trigger. An uncatalogued source is clearly detected in the three images at the position of the optical counterpart first reported by \nJiang et al. (GCN circ. 40052) and also detected by Perez-Garcia et al. (GCN circ. 40053), Du et al. (GCN circ. 40058), Zhu et al. (GCN circ. 40061), Konno et al. (GCN circ. 40063), Odeh et al. (GCN circ. 40064), and He et al. (GCN circ. 40069). A redshift of z = 2.63 has been reported by Zhu et al. (GCN circ. 40061).\n\nWe measure the following preliminary magnitudes, calibrated against Pan-STARRS DR2 stars, that are not corrected for Galactic extinction:\n\n\nDate | UT start | mag | error | filter | exposure time (sec)\n---------------------------------------------------------------------\n2025-04-04 20:27:26 20.11 0.22 i 300\n2025-04-04 22:01:01 20.65 0.16 g 300\n2025-04-04 22:05:55 20.38 0.11 r 300\n\n \nThis work makes use of observations from the Las Cumbres Observatory global telescope network (LCOGT observing programme IAC2025A-009, SGLF).\n\n\n",
"published": "2025-05-10T16:09:50.894754Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.894782Z",
"modified": "2025-05-10T16:09:50.897967Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186286,
"topic": "gcn.circulars",
"uuid": "eacd3127-1937-4d0e-b355-776d364f7c94",
"title": "EP250404a: Mephisto multi-band optical detection",
"submitter": "Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>",
"authors": "Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40058"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40058....1D",
"eventId": "EP250404a",
"subject": "EP250404a: Mephisto multi-band optical detection",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>",
"editedOn": 1743783957962,
"createdOn": 1743783548449,
"submitter": "Chenxu Liu at Mephisto Team <cxliu@ynu.edu.cn>",
"circularId": 40058,
"submittedHow": "web"
},
"message_text": "Guowang Du, Ziwei Li, Chenxu Liu, Dezi Liu, Yaosong Yu, Chenxi Shang, Jinghua Zhang, Yuan Fang, Xufeng Zhu, Tao Wang, Xinlei Chen, Xingzhu Zou, Yu Pan, Brajesh Kumar, Yuanpei Yang, Xiangkun Liu, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:\n\nWe performed simultaneous multi-band photometric observations of EP250404a (EP-WXT ID: 01709133918) detected by EP (Hu et al., GCN 40051) and by Fermi/GBM (Fermi team, GCN 40050) with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at the Lijiang Observatory. The observations were initiated at 14:32:03UTC2025-04-04 (i.e., 723s after the EP/WXT trigger) and are still ongoing at the time of this report. Multiple frames were obtained in uvgriz- bands. The optical counterpart is clearly detected in all bands. The photometry of the initial frames is listed below:\n\nUT start |Band | Exp | Mag (AB)\n--------------------|-----|-------|----------|--------------------------\n2025/4/4 14:32:03 | u | 180s | 18.65+/-0.08\n2025/4/4 14:39:20 | v | 180s | 16.91+/-0.02\n2025/4/4 14:32:05 | g | 50s | 15.72+/-0.01\n2025/4/4 14:39:22 | r | 50s | 14.83+/-0.01\n2025/4/4 14:32:04 | i | 79s | 14.60+/-0.01\n2025/4/4 14:39:21 | z | 79s | 14.03+/-0.01\n\nOur results echo the detection from Nanshan/HMT (Jiang et al., GCN 40052) and BOOTES-4/MET (I. Perez-Garcia et al., GCN 40053). \n----------------------------------------------------------\nMephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.\n----------------------------------------------------------",
"published": "2025-05-10T16:09:49.699197Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.699214Z",
"modified": "2025-05-10T16:09:49.702143Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186293,
"topic": "gcn.circulars",
"uuid": "125e1a81-e962-4d7b-9c27-dca2c97398af",
"title": "EP250404a: Multi-band Optical Observations from UCAS-70 and XL-C14",
"submitter": "qiang xi <xiqiang051@gmail.com>",
"authors": "qiang xi <xiqiang051@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40097"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40097....1Q",
"eventId": "EP250404a",
"subject": "EP250404a: Multi-band Optical Observations from UCAS-70 and XL-C14",
"version": 3,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of qiang xi <xiqiang051@gmail.com>",
"editedOn": 1744033485974,
"createdOn": 1743968004388,
"submitter": "qiang xi <xiqiang051@gmail.com>",
"circularId": 40097,
"submittedHow": "web"
},
"message_text": "Qiang-Xi (UCAS), Yiming-Mao (NAOC), Xinxiang-Sun (NAOC), Ningchen-Sun (NAOC), Junjie-Jin (NAOC), Min-He (NAOC) report:\n\nWe detected an optical counterpart for the field of EP 250404A using two independent facilities: the 70 cm telescope UCAS-70 and the 35 cm telescope XL-C14. The optical counterpart was observed at the coordinates:\n\n **RA = 08h20m14.54s, Dec = +35°31′42.2″.**\n \n Stacked imaging of the field has revealed a clear optical transient. The photometric measurements derived from the stacked images are summarized below. In our analysis, the photometry in the Johnson band was calibrated from APASS data converted using the Lupton (2005) equations, while the SDSS band data were calibrated against the SDSS star catalog. The observation time reported corresponds to the median of the individual exposure times.\n \n| Median Time (UT) | Telescope | Exposure (s) | Filter | Magnitude | Err |\n| ----------------------- | --------- | ------------ | ------ | ---------------- | ---- |\n| 2025-04-04 15:42:35.500 | UCAS-70 | 6×180 | B | 18.73 | 0.24 |\n| 2025-04-04 15:45:44.199 | UCAS-70 | 6×180 | V | 17.89 | 0.12 |\n| 2025-04-04 15:48:52.780 | UCAS-70 | 6×180 | R | 17.18 | 0.14 |\n| 2025-04-04 15:52:01.952 | UCAS-70 | 6×180 | I | 16.76 | 0.13 |\n| 2025-04-04 16:04:43.721 | XL-C14 | 11×120 | g | >18.71( 3-sigma limit) | |\n| 2025-04-04 16:17:19.682 | XL-C14 | 13×120 | r | 17.59 | 0.19 |\n| 2025-04-04 16:15:12.716 | XL-C14 | 13×120 | i | 17.15 | 0.20 |\n| 2025-04-04 15:58:22.729 | XL-C14 | 8×120 | z | >17.41( 3-sigma limit) | |\n \n This measurement has not been corrected for galaxy extinction.\n \n Our observations are consistent with other already reported Jiang et al. (GCN 40052), Perez-Garcia et al. (GCN 40053), Du et al. (GCN 40058), Zhu et al. (GCN 40061), Konno et al. (GCN 40063), Odeh et al. (GCN 40064), Lipunov et al. (GCN 40066), Min-He et al. (GCN 40069), Lipunov et al. (GCN 40070), Perez-Fournon et al. (GCN 40071), Jelinek et al. (GCN 40072), Rakotondrainibe et al. (GCN 40073), Taguchi et al. (GCN 40074), Brivio et al. (GCN 40075), Ruocco (GCN 40076), Martin-Carrillo et al. (GCN 40080), Pavoni et al. (GCN 40083), Li et al. (GCN 40084), Yin et al. (GCN 40085), Pankov et al. (GCN 40086), Chen et al. (GCN 40088), Chen et al. (GCN 40089), Pankov et al. (GCN 40090), Leonini et al. (GCN 40093), Leonini et al(GCN40095).\n \n \n \n \n ",
"published": "2025-05-10T16:09:49.758117Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.758135Z",
"modified": "2025-05-10T16:09:49.761387Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186328,
"topic": "gcn.circulars",
"uuid": "954da41b-13dd-4fce-80da-a5d241adbd40",
"title": "EP250404a: Nanshan 0.43m Telescope optical observatios",
"submitter": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"authors": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40141"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40141....1Y",
"eventId": "EP250404a",
"subject": "EP250404a: Nanshan 0.43m Telescope optical observatios",
"createdOn": 1744551662601,
"submitter": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"circularId": 40141,
"submittedHow": "web"
},
"message_text": "S. Yaqup, J.Z. Liu, A. Iskandar, Y. Zhang, A. Esamdin, C.H. Bai, T.H. Zhong (XAO) report on behalf of optical group of the XAO:\n\nWe observed the field of the EP250404a (Einstein Probe mission, Trigger ID 01709133918) with the 0.43m telescope at Nanshan Station of Xinjiang Astronomical Observatory (XAO). The observations were started at 2025-04-04T14:54:21(UTC), and obtained a series of gri band images.\n\nThe optical afterglow (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; \nZhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al, GCN 40064; He et al., GCN 40069; \nPerez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al, GCN 40073; \nTaguchi et al., GCN 40074; Ruocco et al., GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., \nGCN 40083; Li et al., GCN 40084; Yin et al., GCN 40085; Pankov et al., GCN 40086; Chu Chen et al., \nGCN 40088 and Chen et al., GCN 40089) was clearly detected at position R.A. = 125.0606 deg, DEC = 35.5282 deg J2000.\n\nThe preliminary photometric magnitudes are below: \n\nObservation Start-Time\t T-T0(hr) Exposure\t Filter Mag.\tErr.\n2025-04-04 14:54:21\t 0.583 060s\t g\t 16.914\t0.106\n2025-04-04 14:55:56\t 0.604 060s\t r\t 16.067\t0.076\n2025-04-04 14:57:12\t 0.626 060s\t i\t 15.610\t0.080\n2025-04-04 14:58:29\t 0.648 060s\t g\t 17.090\t0.108\n2025-04-04 14:59:48\t 0.669 060s\t r\t 16.294\t0.072\n2025-04-04 15:01:05\t 0.690 060s\t i\t 15.940\t0.105\n2025-04-04 15:02:22\t 0.712 060s\t g\t 17.484\t0.108\n2025-04-04 15:03:41\t 0.734 060s\t r\t 16.398\t0.071\n2025-04-04 15:04:58\t 0.755 060s\t i\t 15.965\t0.099\n2025-04-04 15:11:08\t 0.858 120s\t i\t 16.388\t0.067\n2025-04-04 15:13:25\t 0.897 120s\t g\t 17.690\t0.106\n2025-04-04 15:15:44\t 0.934 120s\t r\t 17.066\t0.077\n2025-04-04 15:18:01\t 0.972 120s\t i\t 16.496\t0.074\n2025-04-04 15:20:19\t 1.011 120s\t g\t 17.735\t0.096\n2025-04-04 15:22:38\t 1.049 120s\t r\t 17.030\t0.059\n2025-04-04 15:24:54\t 1.087 120s\t i\t 16.736\t0.061\n2025-04-04 15:27:12\t 1.126 120s\t g\t 18.184\t0.105\n2025-04-04 15:29:31\t 1.164 120s\t r\t 17.407\t0.070\n2025-04-04 15:31:48\t 1.202 120s\t i\t 17.033\t0.067\n2025-04-04 15:34:27\t 1.247 200s\t g\t 18.252\t0.090\n2025-04-04 15:52:56\t 1.554 200s\t r\t 17.719\t0.085\n2025-04-04 15:56:33\t 1.615 200s\t i\t 17.332\t0.088\n2025-04-05 16:00:10\t 1.676 200s\t g\t 18.508\t0.116\n2025-04-05 16:03:50\t 1.736 200s\t r\t 18.121\t0.087\n2025-04-05 16:07:27\t 1.796 200s\t i\t 17.742\t0.101\n2025-04-05 16:11:04\t 1.857 200s\t g\t 18.335\t0.093\n2025-04-05 16:14:43\t 1.917 200s\t r\t 17.924\t0.070\n2025-04-05 16:18:19\t 1.978 200s\t i\t 17.569\t0.090\n2025-04-05 16:25:36\t 2.099 200s\t r\t 18.261\t0.081\n2025-04-05 16:36:30\t 2.280 200s\t r\t 18.681\t0.117\n2025-04-05 16:40:06\t 2.341 200s\t i\t 18.319\t0.136\n2025-04-05 16:43:45\t 2.402 200s\t g\t 18.815\t0.129\n2025-04-05 16:47:42\t 2.467 300s\t g\t 18.713\t0.123\n2025-04-05 16:52:59\t 2.555 300s\t r\t 18.799\t0.132\n2025-04-05 16:58:16\t 2.643 300s\t i\t 17.798\t0.094\n2025-04-05 17:03:34\t 2.732 300s\t g\t 19.368\t0.188\n2025-04-05 17:08:52\t 2.820 300s\t r\t 18.608\t0.111\n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction.",
"published": "2025-05-10T16:09:50.091339Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.091357Z",
"modified": "2025-05-10T16:09:50.094709Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186176,
"topic": "gcn.circulars",
"uuid": "d2a12a18-3bdb-4244-9706-152414fb50f7",
"title": "EP250404a: Nanshan 0.43m Telescope optical observatios",
"submitter": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"authors": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40141"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250404a",
"subject": "EP250404a: Nanshan 0.43m Telescope optical observatios",
"createdOn": 1744551662601,
"submitter": "Abdusamatjan Iskandar at XAO,CAS <abudu@xao.ac.cn>",
"circularId": 40141,
"submittedHow": "web"
},
"message_text": "S. Yaqup, J.Z. Liu, A. Iskandar, Y. Zhang, A. Esamdin, C.H. Bai, T.H. Zhong (XAO) report on behalf of optical group of the XAO:\n\nWe observed the field of the EP250404a (Einstein Probe mission, Trigger ID 01709133918) with the 0.43m telescope at Nanshan Station of Xinjiang Astronomical Observatory (XAO). The observations were started at 2025-04-04T14:54:21(UTC), and obtained a series of gri band images.\n\nThe optical afterglow (Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; \nZhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al, GCN 40064; He et al., GCN 40069; \nPerez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al, GCN 40073; \nTaguchi et al., GCN 40074; Ruocco et al., GCN 40076; Martin-Carrillo et al., GCN 40080; Pavoni et al., \nGCN 40083; Li et al., GCN 40084; Yin et al., GCN 40085; Pankov et al., GCN 40086; Chu Chen et al., \nGCN 40088 and Chen et al., GCN 40089) was clearly detected at position R.A. = 125.0606 deg, DEC = 35.5282 deg J2000.\n\nThe preliminary photometric magnitudes are below: \n\nObservation Start-Time\t T-T0(hr) Exposure\t Filter Mag.\tErr.\n2025-04-04 14:54:21\t 0.583 060s\t g\t 16.914\t0.106\n2025-04-04 14:55:56\t 0.604 060s\t r\t 16.067\t0.076\n2025-04-04 14:57:12\t 0.626 060s\t i\t 15.610\t0.080\n2025-04-04 14:58:29\t 0.648 060s\t g\t 17.090\t0.108\n2025-04-04 14:59:48\t 0.669 060s\t r\t 16.294\t0.072\n2025-04-04 15:01:05\t 0.690 060s\t i\t 15.940\t0.105\n2025-04-04 15:02:22\t 0.712 060s\t g\t 17.484\t0.108\n2025-04-04 15:03:41\t 0.734 060s\t r\t 16.398\t0.071\n2025-04-04 15:04:58\t 0.755 060s\t i\t 15.965\t0.099\n2025-04-04 15:11:08\t 0.858 120s\t i\t 16.388\t0.067\n2025-04-04 15:13:25\t 0.897 120s\t g\t 17.690\t0.106\n2025-04-04 15:15:44\t 0.934 120s\t r\t 17.066\t0.077\n2025-04-04 15:18:01\t 0.972 120s\t i\t 16.496\t0.074\n2025-04-04 15:20:19\t 1.011 120s\t g\t 17.735\t0.096\n2025-04-04 15:22:38\t 1.049 120s\t r\t 17.030\t0.059\n2025-04-04 15:24:54\t 1.087 120s\t i\t 16.736\t0.061\n2025-04-04 15:27:12\t 1.126 120s\t g\t 18.184\t0.105\n2025-04-04 15:29:31\t 1.164 120s\t r\t 17.407\t0.070\n2025-04-04 15:31:48\t 1.202 120s\t i\t 17.033\t0.067\n2025-04-04 15:34:27\t 1.247 200s\t g\t 18.252\t0.090\n2025-04-04 15:52:56\t 1.554 200s\t r\t 17.719\t0.085\n2025-04-04 15:56:33\t 1.615 200s\t i\t 17.332\t0.088\n2025-04-05 16:00:10\t 1.676 200s\t g\t 18.508\t0.116\n2025-04-05 16:03:50\t 1.736 200s\t r\t 18.121\t0.087\n2025-04-05 16:07:27\t 1.796 200s\t i\t 17.742\t0.101\n2025-04-05 16:11:04\t 1.857 200s\t g\t 18.335\t0.093\n2025-04-05 16:14:43\t 1.917 200s\t r\t 17.924\t0.070\n2025-04-05 16:18:19\t 1.978 200s\t i\t 17.569\t0.090\n2025-04-05 16:25:36\t 2.099 200s\t r\t 18.261\t0.081\n2025-04-05 16:36:30\t 2.280 200s\t r\t 18.681\t0.117\n2025-04-05 16:40:06\t 2.341 200s\t i\t 18.319\t0.136\n2025-04-05 16:43:45\t 2.402 200s\t g\t 18.815\t0.129\n2025-04-05 16:47:42\t 2.467 300s\t g\t 18.713\t0.123\n2025-04-05 16:52:59\t 2.555 300s\t r\t 18.799\t0.132\n2025-04-05 16:58:16\t 2.643 300s\t i\t 17.798\t0.094\n2025-04-05 17:03:34\t 2.732 300s\t g\t 19.368\t0.188\n2025-04-05 17:08:52\t 2.820 300s\t r\t 18.608\t0.111\n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitudes are not corrected for Galactic extinction.",
"published": "2025-05-10T16:09:47.889403Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:47.889421Z",
"modified": "2025-05-10T16:09:47.892493Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186312,
"topic": "gcn.circulars",
"uuid": "d0315af8-66fa-4b02-b38a-d5674fb0ae09",
"title": "EP250404a: Nanshan/HMT optical counterpart detection",
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"authors": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40052"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40052....1J",
"eventId": "EP250404a",
"subject": "EP250404a: Nanshan/HMT optical counterpart detection",
"createdOn": 1743779534553,
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"circularId": 40052,
"submittedHow": "web"
},
"message_text": "S.Q. Jiang, Z.P. Zhu, J. An, X. Liu, D. Xu (NAOC), S.Y. Fu (HUST), X. Gao (Urumqi No.1 Senior High School), J.Z. Liu (XAO) report:\n\nWe observed the field of EP250404a detected by EP (Hu et al., GCN 40051), which is temporally and spatially consistent with GRB 250404A detected by Fermi/GBM (GBM team, GCN 40050), using the HMT-0.5m telescope located at Nanshan, Xinjiang, China. Observations started at 14:24:59 UT on 2025-04-04, i.e., 300 s after the EP/WXT trigger time, and a series of unfiltered frames with different exposures were obtained.\n\nAn uncatalogued and varying optical source is detected within the EP/FXT error circle at coordinates:\n\nR.A.(J2000) = 08:20:14.54\nDec.(J2000) = +35:31:41.57\n\nwith an uncertainty of ~0.5 arcsec. The source has R ~ 16.1 mag in the first image, calibrated with the nearby Pan-STARRS field and not corrected for Galactic extinction.\n\nObservations are still ongoing.",
"published": "2025-05-10T16:09:49.920694Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.920711Z",
"modified": "2025-05-10T16:09:49.924010Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186480,
"topic": "gcn.circulars",
"uuid": "5d456790-7152-4a60-a53d-05499c81a687",
"title": "EP250404a: Osservatorio Astronomico \"Nastro Verde\" optical observations: detection of an optical counterpart",
"submitter": "Nello Ruocco at Osservatorio Nastro Verde - Sorrento (Naples) - Italy - MPC Code C82 <osservatorionastroverde@gmail.com>",
"authors": "Nello Ruocco at Osservatorio Nastro Verde - Sorrento (Naples) - Italy - MPC Code C82 <osservatorionastroverde@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40076"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40076....1R",
"eventId": "EP250404a",
"subject": "EP250404a: Osservatorio Astronomico \"Nastro Verde\" optical observations: detection of an optical counterpart",
"createdOn": 1743847539875,
"submitter": "Nello Ruocco at Osservatorio Nastro Verde - Sorrento (Naples) - Italy - MPC Code C82 <osservatorionastroverde@gmail.com>",
"circularId": 40076,
"submittedHow": "web"
},
"message_text": "Nello Ruocco at Osservatorio Nastro Verde - Sorrento (Naples) - Italy\nin a large collaboration with:\nM.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy) and K. Kalinowski (Aarhus University, Department of Physics and Astronomy) report:\n \nFollowing the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission triggered EP-WXT (ID: 01709133918) (GCN Circular 40051 J. W. Hu (NAO, CAS), Q. C. Liu (THU), B. B. Zhang (BNU), Y. L. Hua (PMO, CAS), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team), I pointed at the coordinates RA(J2000)= 08:20:14.59 : Dec(J2000)= +35:31:42.3 and started observations with telescope of Nastro Verde Observatory - Sorrento (Naples), Italy.\nMember of: \nAAVSO - American Association of Variable Star Observers.\nGRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani.\n\nClear sky but moon presence at about 20 degrees from the target.\nThe observations started at 20:36 UT of 2025/04/04, after about 6 hours after the GRB trigger, with clear sky, with principal telescope SC 0.35 f/10 with focal reduced + CCD Sbig ST10 XME\nI took 22 unfiltered images of 120 sec . \n Start End Rlim\n20:36:23 UT 21:22:15 UT 20.5\n\nAll images, calibrated with masterdark and masterflat have been measured with Thycho Tracker software\nWe have detected a faint source at the enhanced position reported by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission AT following position\n\nRA (J2000.0) 08 20 15.95\nDec (J2000.0) +35 31 48.3 \n\nwith the following photometry and astrometry:\n\n\nEP250404a 08 20 15.95 +35 31 48.3 19.0 C82\n\n\n\n\nMagnitudes were estimated with the ATLAS cat. and are not corrected for galactic dust extinction.\n\n\n\nThe message may be cited.",
"published": "2025-05-10T16:09:51.548160Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.548177Z",
"modified": "2025-05-10T16:09:51.551516Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186257,
"topic": "gcn.circulars",
"uuid": "1d4df67a-8802-46b4-b681-938652a3dad1",
"title": "EP250404a: refined analysis of the EP-WXT and EP-FXT observations",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40085"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40085....1Y",
"eventId": "EP250404a",
"subject": "EP250404a: refined analysis of the EP-WXT and EP-FXT observations",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"editedOn": 1744033515625,
"createdOn": 1743932707339,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40085,
"submittedHow": "web"
},
"message_text": "Y.-H. I. Yin (NJU), J. W. Hu (NAO, CAS), Y. L. Hua (PMO, CAS), Q. C. Liu (THU), B. B. Zhang (BNU), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n\nThe X-ray transient EP250404a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Hu et al., GCN 40051), and followed up by several telescopes (Jiang et al, GCN 40052, Perez-Garcia et al, GCN 40053, Du et al, GCN 40058, Zhu et al, GCN 40061, Konno et al, GCN 40063, Odeh et al, GCN 40064, Lipunov et al, GCN 40066, He et al, GCN 40069, Pérez-Fournon et al, GCN 40071, Jelinek et al, GCN 40072, Rakotondrainibe et al, GCN 40073, Taguchi et al, GCN 40074, Brivio et al, GCN 40075, Ruocco et al, GCN 70076, Martin-Carrillo et al, GCN 40080, Pavoni et al, GCN 40083, Li et al, GCN 40084), with an optical counterpart detected at a redshift of 2.63 (Zhu et al, GCN 40061, Li et al, GCN 40084). Refined analysis of the WXT data shows that the event started at T0=2025-04-04T14:19:45 (UTC) and lasted for about 16 s before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.41 (-0.28/+0.26). The absorption is fixed at the best-fit values from FXT spectral fitting with a Galactic hydrogen column density of 6.00 x 10^20 cm^-2 and an intrinsic hydrogen column density of 4.89 x 10^22 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 5.9 (-1.6/+2.2) x 10^(-8) erg/s/cm^2. The data recovered from the slew process shows that the transient event lasted for about 300 seconds.\n\nThe Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously about 1 min after T0, starting at 2025-04-04T14:20:46 (UTC). Within the WXT error circle, on-ground analysis of the FXT data found an uncatalogued source at R.A. = 125.0601, DEC = 35.5284 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is also consistent with the position of the optical counterpart. The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 6.00 x 10^20 cm^-2 and an intrinsic hydrogen column density of 4.89 (-0.45/+0.33) x 10^22 cm^-2. The photon index is 2.18 (-0.03/+0.03). The derived average unabsorbed 0.5-10 keV flux is 9.00 (-0.25/+0.19) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:49.452256Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.452273Z",
"modified": "2025-05-10T16:09:49.456272Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186353,
"topic": "gcn.circulars",
"uuid": "a8d49fc9-9037-48c1-a2f7-a1282b31167f",
"title": "EP250404a: REM optical/NIR observations",
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"authors": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40075"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40075....1B",
"eventId": "EP250404a",
"subject": "EP250404a: REM optical/NIR observations",
"createdOn": 1743845561961,
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"circularId": 40075,
"submittedHow": "web"
},
"message_text": "R. Brivio, M. Ferro, P. D'Avanzo, S. Covino, D. Fugazza (INAF-OAB) report on behalf of the REM team:\n\nWe observed the field of EP250404a (Hu et al., GCN 40051), likely associated with GRB 250404A detected by Fermi/GBM (Fermi GBM team, GCN 40050) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, z, J, H, and K bands, started on 2025 April 04 at 23:26:12 UT (i.e. 9.1 hr after the burst), and lasted for about 1 hour.\n\nFrom preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (Jiang et al., GCN. 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058; Zhu et al., GCN 40061; Konno et al., GCN 40063; Odeh et al., GCN 40064; He et al., GCN 40069; Pérez-Fournon et al., GCN 40071; Jelinek et al., GCN 40072; Rakotondrainibe et al., GCN 40073; Taguchi et al., GCN 40074) down to the following 3sigma limits:\n\nr > 19.7 (AB; calibrated against the Pan-STARRS catalogue),\nat a mid-time of 9.6 hours after the trigger;\n\nH > 16.3 (Vega; calibrated against the 2MASS catalogue),\nat a mid-time of 9.5 hours after the trigger.",
"published": "2025-05-10T16:09:50.320825Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.320843Z",
"modified": "2025-05-10T16:09:50.323902Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186255,
"topic": "gcn.circulars",
"uuid": "8457af58-8796-416d-9973-145cbc2dfc8c",
"title": "EP250404a: Seimei/TriCCS optical counterpart detection",
"submitter": "Kenta Taguchi <kentagch@kusastro.kyoto-u.ac.jp>",
"authors": "Kenta Taguchi <kentagch@kusastro.kyoto-u.ac.jp>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40074"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40074....1T",
"eventId": "EP250404a",
"subject": "EP250404a: Seimei/TriCCS optical counterpart detection",
"createdOn": 1743844193436,
"submitter": "Kenta Taguchi <kentagch@kusastro.kyoto-u.ac.jp>",
"circularId": 40074,
"submittedHow": "web"
},
"message_text": "Kenta Taguchi, Keiichi Maeda (Kyoto U.), Masaomi Tanaka (Tohoku U.) report on behalf of a larger collaboration:\n\nWe observed the field of EP250404a detected by Einstein Probe (Hu et al., GCN 40051) using the Tricolor CMOS Camera and Spectrograph (TriCCS) on the 3.8-m Seimei telescope. Observations started at 15:27:48 UT on 2025-04-04 (MJD = 60769.64). We detected a counterpart at a position consistent with the source detected by Jiang et al. (GCN 40052, also reported by Perez-Garcia et al. GCN 40053, Du et al. GCN 40058, Konno et al. GCN 40063, Odeh et al. GCN 40064, He et al. GCN 40069, Pérez-Fournon et al. GCN 40071, Jelinek et al. GCN 40072, Rakotondrainibe et al. GCN 40073).\n\nPreliminary analysis gives the following magnitudes of the source:\ng = 18.20 +- 0.05\nr = 17.46 +- 0.04\ni = 17.10 +- 0.03\n",
"published": "2025-05-10T16:09:49.435311Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.435329Z",
"modified": "2025-05-10T16:09:49.438419Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186413,
"topic": "gcn.circulars",
"uuid": "b085889c-c1b6-4627-a36b-e4d35e4c34ce",
"title": "EP250404a: Xinglong-2.16m spectroscopic redshift",
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"authors": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40061"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40061....1Z",
"eventId": "EP250404a",
"subject": "EP250404a: Xinglong-2.16m spectroscopic redshift",
"createdOn": 1743789948416,
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"circularId": 40061,
"submittedHow": "web"
},
"message_text": "Z.P. Zhu, J. An, X. Liu, S.Q. Jiang, J.J. Jin, Z. Fan, D. Xu (NAOC), S.Y. Fu (HUST), report on behalf of a large collaboration:\n\nWe carried out spectroscopy of the optical counterpart (e.g., Jiang et al., GCN 40052; Perez-Garcia et al., GCN 40053; Du et al., GCN 40058) of EP250404a (Hu et al., GCN 40051), likely the identical event to GRB 250404A (GBM team, GCN 40050), using the 2.16-m telescope equipped with the BFOSC instrument, located at Xinglong, Hebei, China. Our spectra cover the wavelength range 3800 - 9000 AA, and consist of 2 exposures of 1800 s each. \n\nThe continuum is throughout the spectrum. From the detection of a broad Lya absorption feature at ~ 4400 AA and relatively weak absorption features, which interpreted as being due to Si IV and Al III, we infer a common redshift of z = 2.63. We conclude this is the likely redshift of the event.\n\nAn intervening system is also detected with absorption features, interpreted as being due to Mg II and Fe II, we inferred its redshift of z = 1.88. \n\nWe acknowledge the expert support from the Xinglong-2.16m staff, in particular Junjun Jia and Jie Zheng.",
"published": "2025-05-10T16:09:50.877713Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.877731Z",
"modified": "2025-05-10T16:09:50.881055Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186446,
"topic": "gcn.circulars",
"uuid": "5258a325-baaa-4ba8-8325-a8ffb61d1f66",
"title": "EP250404a: Xinglong decaying optical counterpart",
"submitter": "Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>",
"authors": "Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40069"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40069....1M",
"eventId": "EP250404a",
"subject": "EP250404a: Xinglong decaying optical counterpart",
"createdOn": 1743805032460,
"submitter": "Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong@nao.cas.cn>",
"circularId": 40069,
"submittedHow": "web"
},
"message_text": "Min-He (NAOC), Yiming-Mao (NAOC), Junjie-Jin (NAOC), Haiyang-Mu (NAOC), Jinlei-Zhang (NAOC), Junjun-Jia (NAOC), Pengliang-Du (NAOC), Zhou-Fan (NAOC), Hong-Wu (NAOC) report:\n\nWe observed the field of the X-ray transient, EP 250404a, detected by EP/WXT using the Xinglong 0.8-m, 2.16-m telescope located at Xinglong, Hebei, China. We obtained 6 x 300 s g'-band frames for 0.8-m telescope, 5 * 300s clear-band frames for 2.16-m telescope during the time between 2025-04-04T14:32:21.91 and 2025-04-04T17:08:42 as soon as the EP trigger. We have found the optical counterpart of EP 250404a in these images, and the light of this object had been decreased rapidly. The magnitude of this object firstly be ~ 15.68+/- 0.014 at 2025-04-04T14:32:21.91 with 0.8-m telescope in g'-band, and then decrease to ~ 19.7+/-0.102 with 2.16-m telescope at a median time of 2025-04-04T17:03:42, which measured by a stacked clear-band image and calibrated with the Pan-STARRS r' band field. The stacked clear-band image using the 2.16-m telescope has a 5-sigma limiting magnitude of ~21.64 mag, calibrated with the Pan-STARRS r'-band field. \nWithin the 20 arcsec error circle of the FXT, we identified an optical transient (OT) localized at R. A. = 08:20:11.41 , Dec. = 35:31:27.68 (J2000).\n\nWe summarize our observation results as follows:\nObs. No. | Start Time (UTC) | End Time (UTC) | Exposure Time (s) | Filter | Apparent mag (AB) |Telescope Name\n1 | 2025-04-04 14:32:21.91 | 2025-04-04 14:37:21.91 | 300s | g' | 15.681 +/- 0.014 | Xinglong 0.8-m Telescope.\n2 | 2025-04-04 14:37:27.81 | 2025-04-04 14:42:27.81 | 300s | g' | 15.717 +/- 0.013 | Xinglong 0.8-m Telescope.\n3 | 2025-04-04 14:42:33.76 | 2025-04-04 14:47:33.76 | 300s | g' | 16.124 +/- 0.019 | Xinglong 0.8-m Telescope.\n4 | 2025-04-04 15:11:06.35 | 2025-04-04 15:16:06.35 | 300s | g' | 17.864 +/- 0.070 | Xinglong 0.8-m Telescope.\n5 | 2025-04-04 15:16:12.26 | 2025-04-04 15:21:12.26 | 300s | g' | 18.149 +/- 0.088 | Xinglong 0.8-m Telescope.\n6 | 2025-04-04 15:21:18.16 | 2025-04-04 15:26:18.16 | 300s | g' | 19.015 +/- 0.112 | Xinglong 0.8-m Telescope.\n7 | 2025-04-04 14:37:24 | 2025-04-04 14:42:24 | 300s |clear| 14.882 +/- 0.001 (calibrated to r') | Xinglong 2.16-m Telescope.\n8 | 2025-04-04 14:42:32 | 2025-04-04 14:47:32 | 300s |clear| 15.363 +/- 0.002 (calibrated to r')| Xinglong 2.16-m Telescope.\n9 | 2025-04-04 16:58:22 | 2025-04-04 17:13:42 | 3x300s |clear| 19.703 +/- 0.102 (calibrated to r')| Xinglong 2.16-m Telescope \nFurther observations are encouraged.\n",
"published": "2025-05-10T16:09:51.200929Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.200947Z",
"modified": "2025-05-10T16:09:51.204684Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186322,
"topic": "gcn.circulars",
"uuid": "515b4f43-12b9-4527-ae57-c3bccba74b1f",
"title": "EP250407a: EP-WXT detection of a fast X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40118"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40118....1H",
"eventId": "EP250407a",
"subject": "EP250407a: EP-WXT detection of a fast X-ray transient",
"createdOn": 1744103616893,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40118,
"submittedHow": "web"
},
"message_text": "D. F. Hu, T. C. Zheng (PMO, CAS), X. Mao, W. X. Wang, W. D. Zhang (NAO, CAS) report on behalf of the Einstein Probe team:\n\nWe report on the detection of a fast X-ray transient EP250407a detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2025-04-07T23:27:23 (UTC) and lasted for about 70s. We apologize that an incorrect trigger date was reported in the GCN Notice (trigger ID: 11916648441). The WXT position of EP250407a is R.A.= 134.813 deg, DEC = -38.956 deg (J2000) with an uncertainty of 2.5 arcmin (radius, 90% C.L. statistical and systematic). No known X-ray sources are found within the WXT error circle. The average 0.5-4 keV spectrum can be fitted with an absorbed powerlaw with a photon index of 1.4 (+/-0.6) and a fixed Galactic hydrogen column density of 4.67 x 10^21 cm^-2. We derive an average unabsorbed flux of 4.7(-1.0/+1.2) x 10^(-9) erg/s/cm^2, and a peak flux of approximately 1.0 x 10^(-8) erg/s/cm^2 (both in 0.5-4 keV).\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP is planned. Further follow-up observations are encouraged to identify the nature of this X-ray transient. \n\nThe contact TA of this source is Ding-Fang Hu, please contact him via the email dfhu@pmo.ac.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:50.030807Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.030825Z",
"modified": "2025-05-10T16:09:50.034049Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186418,
"topic": "gcn.circulars",
"uuid": "74f9ee4d-5a37-47c6-9168-86955e14fd0f",
"title": "EP250407a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40126"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40126....1L",
"eventId": "EP250407a",
"subject": "EP250407a: Global MASTER-Net observations report",
"createdOn": 1744154872743,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40126,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250407a ( EP Team et al., GCN 40118) errorbox 49972 sec after notice time and 85240 sec after trigger time at 2025-04-08 23:08:03 UT, with upper limit up to 18.5 mag. Observations started at twilight. The observations began at zenith distance = 18 deg. The sun altitude is -10.6 deg. \n\nThe galactic latitude b = 5 deg., longitude l = 262 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2836941\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 85271 | 2025-04-08 23:08:03 | MASTER-OAFA | (08h 59m 01.05s , -38d 53m 39.6s) | C | 60 | 18.2 | \n 85448 | 2025-04-08 23:11:01 | MASTER-OAFA | (08h 59m 05.07s , -38d 53m 10.9s) | C | 60 | 18.5 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:50.921645Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.921663Z",
"modified": "2025-05-10T16:09:50.924913Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186321,
"topic": "gcn.circulars",
"uuid": "275e53f0-08d2-480d-ad6f-323213d05bff",
"title": "EP250407a: Swift-XRT observations",
"submitter": "Maia Williams at PSU <mjw6837@psu.edu>",
"authors": "Maia Williams at PSU <mjw6837@psu.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40123"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40123....1W",
"eventId": "EP250407a",
"subject": "EP250407a: Swift-XRT observations",
"createdOn": 1744140026877,
"submitter": "Maia Williams at PSU <mjw6837@psu.edu>",
"circularId": 40123,
"submittedHow": "web"
},
"message_text": "M. A. Williams (PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team:\n\nWe have analyzed 2100 s of XRT data for the recently reported transient EP250407a (GCN #40118), from T0+36.5 ks to T0+42.4 ks after detection by Einstein Probe WXT. The data are entirely in Photon Counting (PC) mode.\n\nNo X-ray sources have been detected within the estimated 90% EP-WXT error region (2.5 arcmin), with an upper limit on count rate of 5.3 × 10^-3 ct/s. For a typical GRB-like spectrum, this equates to an upper limit on flux of 2 x 10^-13 erg/s/cm^2 (0.3-10 keV).\n\nThe results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/EP/EP_FIELD00033/.",
"published": "2025-05-10T16:09:50.022428Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.022446Z",
"modified": "2025-05-10T16:09:50.025943Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186300,
"topic": "gcn.circulars",
"uuid": "b5c29b9d-35c2-46a8-b548-b353ad3c6f94",
"title": "EP250416a: Einstein Probe detection of an X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40154"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40154....1Z",
"eventId": "EP250416a",
"subject": "EP250416a: Einstein Probe detection of an X-ray transient",
"createdOn": 1744859300031,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40154,
"submittedHow": "web"
},
"message_text": "G.Y. Zhao (SYSU), C. Zhou (HUST), X.L. Chen, K. Chatterjee (YNU), H. Zhou (PMO, CAS) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n \nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250416a. The transient triggered EP-WXT (ID: 01709135021) at 2025-04-16T17:53:59 (UTC). The WXT position of the source is R.A. = 256.402 deg, DEC = 25.768 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).\n \nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 256.4228 deg, DEC = 25.7755 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).\n \nFurther information will be updated when the telemetry data is received.\n \nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:49.817553Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:49.817571Z",
"modified": "2025-05-10T16:09:49.820674Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186191,
"topic": "gcn.circulars",
"uuid": "d0279249-492c-4ff3-9488-733f45681f77",
"title": "EP250416a: Einstein Probe detection of an X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40154"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: Einstein Probe detection of an X-ray transient",
"createdOn": 1744859300031,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40154,
"submittedHow": "web"
},
"message_text": "G.Y. Zhao (SYSU), C. Zhou (HUST), X.L. Chen, K. Chatterjee (YNU), H. Zhou (PMO, CAS) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n \nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250416a. The transient triggered EP-WXT (ID: 01709135021) at 2025-04-16T17:53:59 (UTC). The WXT position of the source is R.A. = 256.402 deg, DEC = 25.768 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).\n \nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 256.4228 deg, DEC = 25.7755 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic).\n \nFurther information will be updated when the telemetry data is received.\n \nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:48.638235Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:48.638253Z",
"modified": "2025-05-10T16:09:48.641395Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186197,
"topic": "gcn.circulars",
"uuid": "743e4bf5-d5b1-4ee1-b4b3-318cab4c99f9",
"title": "EP250416a: Gemini South-GMOS optical counterpart",
"submitter": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"authors": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40160"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: Gemini South-GMOS optical counterpart",
"createdOn": 1744886572137,
"submitter": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"circularId": 40160,
"submittedHow": "web"
},
"message_text": "Andrew J. Levan (Radboud), Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz E. Bauer (UTA), Antonio Martin-Carrillo (UCD), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud), Gregory Corcoran (UCD) report on behalf of a larger collaboration:\n\n\nWe observed the field of the Einstein Probe (EP) transient EP250416a (Zhao et al., GCN 40154) using the Gemini South-GMOS in imaging mode located on Cerro Pachón, Chile. \n\nPhotometric observations started at 2025-04-17 06:51:03 UTC (i.e., ~0.54 days after the EP detection), consisting of 4 exposures of 60 s each, carried out using filter r'.\n\nConsistent with the position of the X-ray source detected by XRT (target ID 19721, PI J. Kennea; https://www.swift.ac.uk/EP/) and the EP/FXT localization, we notice a source at coordinates measured from the WCS-calibrated Gemini image:\n\nRA = 17:05:40.83\nDec = +25:46:31.5\n\nThis source had magnitude r = 22.8 +/- 0.1 (AB), calibrated against the Pan-STARRS objects in the field. An underlying, fainter source is catalogued in the Legacy Survey (DR10; Dey et al. 2019), with a quoted magnitude of r = 23.16 and photometric redshift of 0.72+-0.24 (Zhou et al. 2021).\n\nBased on the position and brighter magnitude relative to the previously catalogued object, we conclude that the source is very likely the optical counterpart of EP250416a.\n\nWe acknowledge excellent support from the Gemini South staff.",
"published": "2025-05-10T16:09:48.692099Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:48.692117Z",
"modified": "2025-05-10T16:09:48.695201Z",
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{
"id": 186454,
"topic": "gcn.circulars",
"uuid": "2175a4c0-c2ed-4906-89f0-0d4e69712a7a",
"title": "EP250416a: Gemini South-GMOS optical counterpart",
"submitter": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"authors": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40160"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40160....1L",
"eventId": "EP250416a",
"subject": "EP250416a: Gemini South-GMOS optical counterpart",
"createdOn": 1744886572137,
"submitter": "Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>",
"circularId": 40160,
"submittedHow": "web"
},
"message_text": "Andrew J. Levan (Radboud), Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz E. Bauer (UTA), Antonio Martin-Carrillo (UCD), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud), Gregory Corcoran (UCD) report on behalf of a larger collaboration:\n\n\nWe observed the field of the Einstein Probe (EP) transient EP250416a (Zhao et al., GCN 40154) using the Gemini South-GMOS in imaging mode located on Cerro Pachón, Chile. \n\nPhotometric observations started at 2025-04-17 06:51:03 UTC (i.e., ~0.54 days after the EP detection), consisting of 4 exposures of 60 s each, carried out using filter r'.\n\nConsistent with the position of the X-ray source detected by XRT (target ID 19721, PI J. Kennea; https://www.swift.ac.uk/EP/) and the EP/FXT localization, we notice a source at coordinates measured from the WCS-calibrated Gemini image:\n\nRA = 17:05:40.83\nDec = +25:46:31.5\n\nThis source had magnitude r = 22.8 +/- 0.1 (AB), calibrated against the Pan-STARRS objects in the field. An underlying, fainter source is catalogued in the Legacy Survey (DR10; Dey et al. 2019), with a quoted magnitude of r = 23.16 and photometric redshift of 0.72+-0.24 (Zhou et al. 2021).\n\nBased on the position and brighter magnitude relative to the previously catalogued object, we conclude that the source is very likely the optical counterpart of EP250416a.\n\nWe acknowledge excellent support from the Gemini South staff.",
"published": "2025-05-10T16:09:51.285046Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.285065Z",
"modified": "2025-05-10T16:09:51.288035Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186193,
"topic": "gcn.circulars",
"uuid": "cf4747e8-bbbf-4d33-9691-43b8c481e1a8",
"title": "EP250416a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
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"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: Global MASTER-Net observations report",
"createdOn": 1744867112111,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40156,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250416a ( EP Team et al., GCN 40154) errorbox 3581 sec after notice time and 37096 sec after trigger time at 2025-04-17 04:12:15 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 79 deg. The sun altitude is -68.8 deg. \n\nThe galactic latitude b = 33 deg., longitude l = 47 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2845513\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 37127 | 2025-04-17 04:12:15 | MASTER-OAFA | (17h 05m 52.87s , +25d 40m 55.7s) | C | 60 | 16.5 | \n 37187 | 2025-04-17 04:12:15 | MASTER-OAFA | (17h 05m 52.85s , +25d 40m 55.5s) | C | 180 | 17.7 | Coadd \n 37590 | 2025-04-17 04:19:58 | MASTER-OAFA | (17h 05m 56.72s , +25d 41m 13.9s) | C | 60 | 17.3 | \n 38093 | 2025-04-17 04:28:21 | MASTER-OAFA | (17h 06m 00.04s , +25d 40m 57.5s) | C | 60 | 17.5 | \n 38511 | 2025-04-17 04:35:20 | MASTER-OAFA | (17h 05m 58.06s , +25d 40m 40.2s) | C | 60 | 17.7 | \n 39442 | 2025-04-17 04:50:50 | MASTER-OAFA | (17h 06m 00.53s , +25d 42m 42.8s) | C | 60 | 18.2 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:48.655303Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:48.655321Z",
"modified": "2025-05-10T16:09:48.658658Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186453,
"topic": "gcn.circulars",
"uuid": "4ce352a3-d7e5-4431-9464-ffcce6ecb16b",
"title": "EP250416a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
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},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40156....1L",
"eventId": "EP250416a",
"subject": "EP250416a: Global MASTER-Net observations report",
"createdOn": 1744867112111,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40156,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250416a ( EP Team et al., GCN 40154) errorbox 3581 sec after notice time and 37096 sec after trigger time at 2025-04-17 04:12:15 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 79 deg. The sun altitude is -68.8 deg. \n\nThe galactic latitude b = 33 deg., longitude l = 47 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2845513\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 37127 | 2025-04-17 04:12:15 | MASTER-OAFA | (17h 05m 52.87s , +25d 40m 55.7s) | C | 60 | 16.5 | \n 37187 | 2025-04-17 04:12:15 | MASTER-OAFA | (17h 05m 52.85s , +25d 40m 55.5s) | C | 180 | 17.7 | Coadd \n 37590 | 2025-04-17 04:19:58 | MASTER-OAFA | (17h 05m 56.72s , +25d 41m 13.9s) | C | 60 | 17.3 | \n 38093 | 2025-04-17 04:28:21 | MASTER-OAFA | (17h 06m 00.04s , +25d 40m 57.5s) | C | 60 | 17.5 | \n 38511 | 2025-04-17 04:35:20 | MASTER-OAFA | (17h 05m 58.06s , +25d 40m 40.2s) | C | 60 | 17.7 | \n 39442 | 2025-04-17 04:50:50 | MASTER-OAFA | (17h 06m 00.53s , +25d 42m 42.8s) | C | 60 | 18.2 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:51.277024Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.277043Z",
"modified": "2025-05-10T16:09:51.280311Z",
"nonlocalizedevents": [],
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},
{
"id": 186596,
"topic": "gcn.circulars",
"uuid": "abb834a4-93b4-4d1b-923d-f0f92f75480d",
"title": "EP250416a / GRB 250416C: Gemini GMOS-S likely host galaxy redshift z = 0.963",
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"authors": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40219"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40219....1L",
"eventId": "EP250416a",
"subject": "EP250416a / GRB 250416C: Gemini GMOS-S likely host galaxy redshift z = 0.963",
"createdOn": 1745432368286,
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"circularId": 40219,
"submittedHow": "web"
},
"message_text": "Andrew J. Levan (Radboud), Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz E. Bauer (UTA), Antonio Martin-Carrillo (UCD), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Maria E. Ravasio (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud), Gregory Corcoran (UCD) report on behalf of a larger collaboration:\n\nWe observed the optical and X-ray counterpart (Levan et al., GCN 40160; Sbarrato et al., GCN 40166) of the Einstein Probe (EP) transient EP250416a (Zhao et al., GCN 40154, 40165), also identified as GRB 250416C (Svinkin et al., GCN 40167; Wang et al., GCN 40184). Observations were carried out using the Gemini South telescope located on Cerro Pachon, Chile, equipped with the GMOS-S instrument in spectroscopy mode.\n\nObservations started on 2025-04-17 at 07:44:43 (i.e., ~0.58 days after the EP detection), and consisted of 4 exposures of 900 s each using the B480 grating, covering the wavelength range between ~4000 and 9500 AA.\n\nIn our spectrum, we clearly detect an emission line at 7317 AA. Considering the most likely options for this feature (Halpha, Hbeta, [O III], [O II]), the interpretation more consistent with the data is the (unresolved) [O II] 3727/3729 doublet at z = 0.963. In all other cases, we would expect to see other features which would fall over well-covered regions of the spectrum, which are not observed. We thus suggest that this is the redshift of the host galaxy of EP250416a.\n\nThe spectrum shows tentative absorption features at the expected locations of Mg II (2796, 2803 AA) and Mg I (2852 AA) at z = 0.962, although the signal to noise in the blue region of the spectrum is badly affected by the bright Moon, hence we cannot conclusively determine an absorption redshift.\n\nWe acknowledge excellent support from the Gemini South staff.\n",
"published": "2025-05-10T16:09:52.526626Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.526643Z",
"modified": "2025-05-10T16:09:52.529887Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186524,
"topic": "gcn.circulars",
"uuid": "54fbb2c7-140c-4f58-9193-dac5d6552566",
"title": "EP250416a / GRB 250416C: Gemini GMOS-S likely host galaxy redshift z = 0.963",
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"authors": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40219"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a / GRB 250416C: Gemini GMOS-S likely host galaxy redshift z = 0.963",
"createdOn": 1745432368286,
"submitter": "Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>",
"circularId": 40219,
"submittedHow": "web"
},
"message_text": "Andrew J. Levan (Radboud), Jonathan Quirola-Vásquez (Radboud), Peter G. Jonker (Radboud), Franz E. Bauer (UTA), Antonio Martin-Carrillo (UCD), Daniele B. Malesani (DAWN/ NBI and Radboud), Javi Sánchez-Sierras (Radboud), Maria E. Ravasio (Radboud), Agnes van Hoof (Radboud), Jennifer Chacon (PUC), Joyce van Dalen (Radboud), Gregory Corcoran (UCD) report on behalf of a larger collaboration:\n\nWe observed the optical and X-ray counterpart (Levan et al., GCN 40160; Sbarrato et al., GCN 40166) of the Einstein Probe (EP) transient EP250416a (Zhao et al., GCN 40154, 40165), also identified as GRB 250416C (Svinkin et al., GCN 40167; Wang et al., GCN 40184). Observations were carried out using the Gemini South telescope located on Cerro Pachon, Chile, equipped with the GMOS-S instrument in spectroscopy mode.\n\nObservations started on 2025-04-17 at 07:44:43 (i.e., ~0.58 days after the EP detection), and consisted of 4 exposures of 900 s each using the B480 grating, covering the wavelength range between ~4000 and 9500 AA.\n\nIn our spectrum, we clearly detect an emission line at 7317 AA. Considering the most likely options for this feature (Halpha, Hbeta, [O III], [O II]), the interpretation more consistent with the data is the (unresolved) [O II] 3727/3729 doublet at z = 0.963. In all other cases, we would expect to see other features which would fall over well-covered regions of the spectrum, which are not observed. We thus suggest that this is the redshift of the host galaxy of EP250416a.\n\nThe spectrum shows tentative absorption features at the expected locations of Mg II (2796, 2803 AA) and Mg I (2852 AA) at z = 0.962, although the signal to noise in the blue region of the spectrum is badly affected by the bright Moon, hence we cannot conclusively determine an absorption redshift.\n\nWe acknowledge excellent support from the Gemini South staff.\n",
"published": "2025-05-10T16:09:51.920046Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.920063Z",
"modified": "2025-05-10T16:09:51.923094Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186489,
"topic": "gcn.circulars",
"uuid": "a20b27ac-9ce8-4525-883a-c74dbb06af08",
"title": "EP250416a: GSP optical upper limit",
"submitter": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"authors": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40157"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40157....1L",
"eventId": "EP250416a",
"subject": "EP250416a: GSP optical upper limit",
"createdOn": 1744869973378,
"submitter": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"circularId": 40157,
"submittedHow": "web"
},
"message_text": "W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:\nFollowing the detection of the fast X-ray transient EP250416a by the Einstein Probe (Zhao et al., GCN 40154), we initiated observations of its location starting on 2025 April 17th at 01:43 UT (~8 hours after the EP/WXT trigger) in the i band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Siding Spring Observatory in Australia.\nNo new optical source was detected in the co-added images within the EP/FXT error box down to ~20 mag.\nThese observations were taken as part of the Global Supernova Project.\n",
"published": "2025-05-10T16:09:51.626517Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.626535Z",
"modified": "2025-05-10T16:09:51.629651Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186194,
"topic": "gcn.circulars",
"uuid": "3af02b6d-015f-42de-85ae-719211ce578c",
"title": "EP250416a: GSP optical upper limit",
"submitter": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"authors": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"data": {
"urls": {
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},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: GSP optical upper limit",
"createdOn": 1744869973378,
"submitter": "Wenxiong Li at NAOC <liwenxiong1992@gmail.com>",
"circularId": 40157,
"submittedHow": "web"
},
"message_text": "W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:\nFollowing the detection of the fast X-ray transient EP250416a by the Einstein Probe (Zhao et al., GCN 40154), we initiated observations of its location starting on 2025 April 17th at 01:43 UT (~8 hours after the EP/WXT trigger) in the i band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Siding Spring Observatory in Australia.\nNo new optical source was detected in the co-added images within the EP/FXT error box down to ~20 mag.\nThese observations were taken as part of the Global Supernova Project.\n",
"published": "2025-05-10T16:09:48.664226Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:48.664245Z",
"modified": "2025-05-10T16:09:48.667362Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186205,
"topic": "gcn.circulars",
"uuid": "5608b7af-b667-489e-8153-66021a297f3b",
"title": "EP250416a: refined EP-WXT and EP-FXT analysis",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40165"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: refined EP-WXT and EP-FXT analysis",
"createdOn": 1744939804262,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40165,
"submittedHow": "web"
},
"message_text": "H. Zhou (PMO, CAS), G.Y. Zhao (SYSU), C. Zhou (HUST), X.L. Chen, K. Chatterjee (YNU) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n \nWe reduced and analyzed the telemetry data of EP250416a, and all uncertainties reported here are at the 90% confident level. EP250416a became detectable by WXT from ~30 seconds before the trigger time (2025-04-16T17:53:59 UTC, Zhao et al. GCN 40154), and the WXT light curve lasts for ~30 seconds (interupted by the autonomous follow-up observation) with a peak occuring at ~20 seconds after the time when EP250416a became detectable. Lipunov et al. (GCN 40156) and Li et al. (GCN 40157) performed follow-up observations for EP250416a but did not find new optical transient down to i ~20 mag. Later, a possible candidate was detected by Gemini-South (Levan et al. GCN 40160).\n\nThe WXT spectrum is fitted by an absorbed powerlaw model, which counts for absorption of the Milky Way and the host galaxy. As suggested by Levan et al. (GCN 40160), the redshift of the host is set to 0.72. The equivalent hydrogen column density of the host, NH, is constrained to be less than ~1.9x10^22 cm^-2 by the WXT spectrum. The photon index of the WXT spectrum is hard, 0.32 (-0.78, +1.00). The mean and peak unabsorbed fluxes (0.5-4 keV) are (5.66+/-1.77)x10^-9 erg/s/cm^2 and (1.92+/-0.76)x10^-8 erg/s/cm^2.\n\nThe autonomous EP-FXT follow-up observation starts about 2 minutes after the trigger and lasts for 2 orbits with total on-source time of 3936 seconds. In addition, about 12.3 hours after the trigger, another follow-up observation with the EP-FXT was performed for 2962 seconds.\n\nResults of the autonomous and the follow-up observations are summarized:\n+--------------------------------------------------------------------------------+\n T_Start^a [UTC] | T_mid - T0^b [h] | Exp [s] | Flux (0.5-10 keV) [erg/s/cm^2] \n+--------------------------------------------------------------------------------+\n 2025-04-16T17:54:53 | 0.17 | 929 | (2.56+/-0.17) x 10^-11\n 2025-04-16T18:55:35 | 1.49 | 3007 | (7.86+/-0.48) x 10^-12\n 2025-04-17T06:08:14 | 12.68 | 2962 | (1.64+/-0.20) x 10^-12\n+--------------------------------------------------------------------------------+\nAll fluxes are unabsorbed values derived from FXT-A & FXT-B data.\na. The first and the second rows represent the first and the second orbits of the autonomous observation.\nb. Here T0 is the time that EP250416a became detectable, about 30 seconds before the trigger time.\n\nThe inferred 0.5-10 keV flux at the XRT epoch is consistent the value derived from the XRT observation (Sbarrato et al. GCN 40163). The model, which is same as the model applied to WXT data, is applied to fit FXT spectra. For the three epochs listed in the above table, the best fitted photon indices are 1.88+/-0.14, 1.98+/-0.15 and 2.23+/-0.38, and the NH of the host are (5.06+/-1.74)x10^21 cm^-2, (5.33+/-1.77)x10^21 cm^-2 and (6.66+/-4.54)x10^21 cm^-2.\n\nThe contact TA of EP250416a is Guo-Ying Zhao. Please contact her via email zhaogy28@mail2.sysu.edu.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:48.760967Z",
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"retracted": false,
"created": "2025-05-10T16:09:48.760985Z",
"modified": "2025-05-10T16:09:48.764237Z",
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},
{
"id": 186395,
"topic": "gcn.circulars",
"uuid": "fdb90b62-be1b-4867-a2c2-eac7ef2fe12f",
"title": "EP250416a: refined EP-WXT and EP-FXT analysis",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
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"bibcode": "2025GCN.40165....1Z",
"eventId": "EP250416a",
"subject": "EP250416a: refined EP-WXT and EP-FXT analysis",
"createdOn": 1744939804262,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40165,
"submittedHow": "web"
},
"message_text": "H. Zhou (PMO, CAS), G.Y. Zhao (SYSU), C. Zhou (HUST), X.L. Chen, K. Chatterjee (YNU) and C.C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n \nWe reduced and analyzed the telemetry data of EP250416a, and all uncertainties reported here are at the 90% confident level. EP250416a became detectable by WXT from ~30 seconds before the trigger time (2025-04-16T17:53:59 UTC, Zhao et al. GCN 40154), and the WXT light curve lasts for ~30 seconds (interupted by the autonomous follow-up observation) with a peak occuring at ~20 seconds after the time when EP250416a became detectable. Lipunov et al. (GCN 40156) and Li et al. (GCN 40157) performed follow-up observations for EP250416a but did not find new optical transient down to i ~20 mag. Later, a possible candidate was detected by Gemini-South (Levan et al. GCN 40160).\n\nThe WXT spectrum is fitted by an absorbed powerlaw model, which counts for absorption of the Milky Way and the host galaxy. As suggested by Levan et al. (GCN 40160), the redshift of the host is set to 0.72. The equivalent hydrogen column density of the host, NH, is constrained to be less than ~1.9x10^22 cm^-2 by the WXT spectrum. The photon index of the WXT spectrum is hard, 0.32 (-0.78, +1.00). The mean and peak unabsorbed fluxes (0.5-4 keV) are (5.66+/-1.77)x10^-9 erg/s/cm^2 and (1.92+/-0.76)x10^-8 erg/s/cm^2.\n\nThe autonomous EP-FXT follow-up observation starts about 2 minutes after the trigger and lasts for 2 orbits with total on-source time of 3936 seconds. In addition, about 12.3 hours after the trigger, another follow-up observation with the EP-FXT was performed for 2962 seconds.\n\nResults of the autonomous and the follow-up observations are summarized:\n+--------------------------------------------------------------------------------+\n T_Start^a [UTC] | T_mid - T0^b [h] | Exp [s] | Flux (0.5-10 keV) [erg/s/cm^2] \n+--------------------------------------------------------------------------------+\n 2025-04-16T17:54:53 | 0.17 | 929 | (2.56+/-0.17) x 10^-11\n 2025-04-16T18:55:35 | 1.49 | 3007 | (7.86+/-0.48) x 10^-12\n 2025-04-17T06:08:14 | 12.68 | 2962 | (1.64+/-0.20) x 10^-12\n+--------------------------------------------------------------------------------+\nAll fluxes are unabsorbed values derived from FXT-A & FXT-B data.\na. The first and the second rows represent the first and the second orbits of the autonomous observation.\nb. Here T0 is the time that EP250416a became detectable, about 30 seconds before the trigger time.\n\nThe inferred 0.5-10 keV flux at the XRT epoch is consistent the value derived from the XRT observation (Sbarrato et al. GCN 40163). The model, which is same as the model applied to WXT data, is applied to fit FXT spectra. For the three epochs listed in the above table, the best fitted photon indices are 1.88+/-0.14, 1.98+/-0.15 and 2.23+/-0.38, and the NH of the host are (5.06+/-1.74)x10^21 cm^-2, (5.33+/-1.77)x10^21 cm^-2 and (6.66+/-4.54)x10^21 cm^-2.\n\nThe contact TA of EP250416a is Guo-Ying Zhao. Please contact her via email zhaogy28@mail2.sysu.edu.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:50.718233Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:50.718250Z",
"modified": "2025-05-10T16:09:50.721332Z",
"nonlocalizedevents": [],
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},
{
"id": 186206,
"topic": "gcn.circulars",
"uuid": "7707b5a8-a303-472f-86b3-81659113130f",
"title": "EP250416a: Swift/XRT counterpart fading",
"submitter": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"authors": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40166"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250416a",
"subject": "EP250416a: Swift/XRT counterpart fading",
"createdOn": 1744986677678,
"submitter": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"circularId": 40166,
"submittedHow": "web"
},
"message_text": "T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), D. Malesani (DAWN/NBI and Radboud), K. Page (U. Leicester) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:\n\nSwift-XRT has performed further follow-up observations of the EP-WXT transient EP250416a (Zhao et al., GCN 40154), collecting a total of 4.0 ks of Photon Counting (PC) mode data between T0+17.1 ks and T0+79.8 ks. \n\nThe X-ray source detected in the first 2.0 ks of data reported as “Source 1”, consistent with the optical counterpart (Levan et al., GCN 40160), and presented as the possible transient counterpart (Sbarrato et al., GCN 40163) has faded more than 3 sigma in the latest observation, confirming the trend observed by EP-FXT about 12.3 hours after the trigger (Zhou et al., GCN 40165). \nThe flux derived from the follow-up observation is 1.8 (+/-0.4) x 10^-2 cts/s, that corresponds to 6.5 (+/-1.3) x 10^-13 erg cm^-2 s^-1.\nThe light curve can be modelled with a power-law decay with an index of alpha=1.18 \n\nA spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+0.5, -0.4). The best-fitting absorption column is 1.6 (+1.7, -1.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.65 x 10^-11 (5.01 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from all the PC mode data is thus 1.8 x 10^-12 (2.5 x 10^-12) erg cm^-2 s^-1. \n\nThe results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00034/\n\nThis circular is an official product of the Swift-XRT team.",
"published": "2025-05-10T16:09:48.769200Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:48.769218Z",
"modified": "2025-05-10T16:09:48.772684Z",
"nonlocalizedevents": [],
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},
{
"id": 186626,
"topic": "gcn.circulars",
"uuid": "5566b9e6-28cc-44d3-bd77-6cca8550eaff",
"title": "EP250416a: Swift/XRT counterpart fading",
"submitter": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"authors": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"data": {
"urls": {
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},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40166....1S",
"eventId": "EP250416a",
"subject": "EP250416a: Swift/XRT counterpart fading",
"createdOn": 1744986677678,
"submitter": "Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it>",
"circularId": 40166,
"submittedHow": "web"
},
"message_text": "T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), D. Malesani (DAWN/NBI and Radboud), K. Page (U. Leicester) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:\n\nSwift-XRT has performed further follow-up observations of the EP-WXT transient EP250416a (Zhao et al., GCN 40154), collecting a total of 4.0 ks of Photon Counting (PC) mode data between T0+17.1 ks and T0+79.8 ks. \n\nThe X-ray source detected in the first 2.0 ks of data reported as “Source 1”, consistent with the optical counterpart (Levan et al., GCN 40160), and presented as the possible transient counterpart (Sbarrato et al., GCN 40163) has faded more than 3 sigma in the latest observation, confirming the trend observed by EP-FXT about 12.3 hours after the trigger (Zhou et al., GCN 40165). \nThe flux derived from the follow-up observation is 1.8 (+/-0.4) x 10^-2 cts/s, that corresponds to 6.5 (+/-1.3) x 10^-13 erg cm^-2 s^-1.\nThe light curve can be modelled with a power-law decay with an index of alpha=1.18 \n\nA spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+0.5, -0.4). The best-fitting absorption column is 1.6 (+1.7, -1.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.65 x 10^-11 (5.01 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from all the PC mode data is thus 1.8 x 10^-12 (2.5 x 10^-12) erg cm^-2 s^-1. \n\nThe results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00034/\n\nThis circular is an official product of the Swift-XRT team.",
"published": "2025-05-10T16:09:52.777139Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.777157Z",
"modified": "2025-05-10T16:09:52.780257Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186593,
"topic": "gcn.circulars",
"uuid": "a64dd19c-b2fb-4a29-b140-2f058cf85bd7",
"title": "EP250421A: COLIBRÍ Optical Upper Limit",
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"authors": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40203"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40203....1B",
"eventId": "EP250421A",
"subject": "EP250421A: COLIBRÍ Optical Upper Limit",
"createdOn": 1745309617463,
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"circularId": 40203,
"submittedHow": "web"
},
"message_text": "Rosa L. Becerra (U Roma), Camila Angulo (UNAM), Francis Fortin (IRAP), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Sarah Antier (OCA/IJCLAB), Dalya Akl (AUS), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Antonio de Ugarte Postigo (LAM) and Benjamin Schneider (LAM) report:\n\nWe imaged the field of EP250421A (Zhao et al., GCN Circ. 40198) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-22 UTC.\n\nWe observed from 2025-04-22 04:54 to 06:03 UTC (T+12.6 to T+13.8 hours after the trigger) and obtained 50 minutes of exposure in total in the r filter. Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nAt the position of the optical counterpart (Lee et al., GCN Circ. 40192; Fu et al., GCN Circ. 40193; Liu et al., GCN Circ. 40197), we obtained an upper limit (5-sgima) of:\n\nr > 23.44\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-10T16:09:52.502828Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.502845Z",
"modified": "2025-05-10T16:09:52.505982Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186506,
"topic": "gcn.circulars",
"uuid": "bdd3be7e-3b8c-4fe5-8d4f-ca23d14d1ff9",
"title": "EP250421A: COLIBRÍ Optical Upper Limit",
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"authors": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40203"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421A",
"subject": "EP250421A: COLIBRÍ Optical Upper Limit",
"createdOn": 1745309617463,
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"circularId": 40203,
"submittedHow": "web"
},
"message_text": "Rosa L. Becerra (U Roma), Camila Angulo (UNAM), Francis Fortin (IRAP), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Sarah Antier (OCA/IJCLAB), Dalya Akl (AUS), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Antonio de Ugarte Postigo (LAM) and Benjamin Schneider (LAM) report:\n\nWe imaged the field of EP250421A (Zhao et al., GCN Circ. 40198) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-22 UTC.\n\nWe observed from 2025-04-22 04:54 to 06:03 UTC (T+12.6 to T+13.8 hours after the trigger) and obtained 50 minutes of exposure in total in the r filter. Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nAt the position of the optical counterpart (Lee et al., GCN Circ. 40192; Fu et al., GCN Circ. 40193; Liu et al., GCN Circ. 40197), we obtained an upper limit (5-sgima) of:\n\nr > 23.44\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-10T16:09:51.770041Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.770059Z",
"modified": "2025-05-10T16:09:51.773063Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186501,
"topic": "gcn.circulars",
"uuid": "32e6bdeb-3221-4daf-aad8-522322faa63e",
"title": "EP250421a: Einstein Probe detection of an X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40198"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Einstein Probe detection of an X-ray transient",
"createdOn": 1745296574384,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40198,
"submittedHow": "web"
},
"message_text": "G. Y. Zhao (SYSU), C. Zhou (HUST), R.-Z. Li (YNAO, CAS), W. Chen, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250421a. The transient triggered EP-WXT (ID: 01709135179) at 2025-04-21T16:16:28 (UTC) and WXT detected a rise of about 70 s. The WXT position of the source is R.A. = 172.4130 deg, DEC = -24.6830 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically about 245 s after the EP/WXT trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 172.4201 deg, DEC = -24.6758 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic).\n\nThe WXT spectrum can be fitted by an absorbed power law model with a photon index of 2.91(+2.76, -1.72) and an absorbing column density of 4.1e21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.3 (+14.7,-0.81) e-9 erg/cm^2/s.\n\nThe FXT spectrum can be fitted by an absorbed power law model with a photon index of 2.11 (+0.15, -0.14) and an absorbing column density of 8.3e20 cm^-2. The unabsorbed 0.5-10 keV flux is about 1.39 (+0.07, -1.39) e-11 erg/cm^2/s.\n\nThe contact TA of EP250421a is Chang Zhou. Please contact her via email d202180110@hust.edu.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:51.729246Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.729264Z",
"modified": "2025-05-10T16:09:51.732371Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186592,
"topic": "gcn.circulars",
"uuid": "004090ed-19eb-45be-aa1c-f9fc528f6ccc",
"title": "EP250421a: Einstein Probe detection of an X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40198"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40198....1Z",
"eventId": "EP250421a",
"subject": "EP250421a: Einstein Probe detection of an X-ray transient",
"createdOn": 1745296574384,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40198,
"submittedHow": "web"
},
"message_text": "G. Y. Zhao (SYSU), C. Zhou (HUST), R.-Z. Li (YNAO, CAS), W. Chen, Z. X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250421a. The transient triggered EP-WXT (ID: 01709135179) at 2025-04-21T16:16:28 (UTC) and WXT detected a rise of about 70 s. The WXT position of the source is R.A. = 172.4130 deg, DEC = -24.6830 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic).\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically about 245 s after the EP/WXT trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 172.4201 deg, DEC = -24.6758 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic).\n\nThe WXT spectrum can be fitted by an absorbed power law model with a photon index of 2.91(+2.76, -1.72) and an absorbing column density of 4.1e21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.3 (+14.7,-0.81) e-9 erg/cm^2/s.\n\nThe FXT spectrum can be fitted by an absorbed power law model with a photon index of 2.11 (+0.15, -0.14) and an absorbing column density of 8.3e20 cm^-2. The unabsorbed 0.5-10 keV flux is about 1.39 (+0.07, -1.39) e-11 erg/cm^2/s.\n\nThe contact TA of EP250421a is Chang Zhou. Please contact her via email d202180110@hust.edu.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:52.493971Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.493988Z",
"modified": "2025-05-10T16:09:52.497564Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186503,
"topic": "gcn.circulars",
"uuid": "d531ab71-f378-47dc-8ff8-8398d9b1295e",
"title": "EP250421a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40200"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Global MASTER-Net observations report",
"createdOn": 1745300209483,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40200,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250421a ( EP Team et al., GCN 40198) errorbox 2313 sec after notice time and 46767 sec after trigger time at 2025-04-22 05:15:55 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 43 deg. The sun altitude is -68.7 deg. \n\nThe galactic latitude b = 35 deg., longitude l = 281 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2849886\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 46798 | 2025-04-22 05:15:55 | MASTER-OAFA | (11h 29m 43.36s , -24d 18m 41.7s) | C | 60 | 20.0 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:51.745630Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.745648Z",
"modified": "2025-05-10T16:09:51.748772Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186621,
"topic": "gcn.circulars",
"uuid": "b4970c5d-4b63-4231-b822-a7bfdaa6ed6a",
"title": "EP250421a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40200"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40200....1L",
"eventId": "EP250421a",
"subject": "EP250421a: Global MASTER-Net observations report",
"createdOn": 1745300209483,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40200,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko,\nA.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,\nV.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250421a ( EP Team et al., GCN 40198) errorbox 2313 sec after notice time and 46767 sec after trigger time at 2025-04-22 05:15:55 UT, with upper limit up to 20.0 mag. The observations began at zenith distance = 43 deg. The sun altitude is -68.7 deg. \n\nThe galactic latitude b = 35 deg., longitude l = 281 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2849886\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 46798 | 2025-04-22 05:15:55 | MASTER-OAFA | (11h 29m 43.36s , -24d 18m 41.7s) | C | 60 | 20.0 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:52.735672Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.735702Z",
"modified": "2025-05-10T16:09:52.738852Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186647,
"topic": "gcn.circulars",
"uuid": "1bf1d174-ead0-4aa3-adbb-409aa5e14174",
"title": "EP250421a: JinShan likely optical counterpart",
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"authors": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40193"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40193....1F",
"eventId": "EP250421a",
"subject": "EP250421a: JinShan likely optical counterpart",
"createdOn": 1745261691092,
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"circularId": 40193,
"submittedHow": "web"
},
"message_text": "S.Y. Fu (HUST), S.Q. Jiang, J. An, Z.P. Zhu, X. Liu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:\n\nWe observed the field of EP250421a (ID: 01709135179) detected by EP/WXT, using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 16:25:07.32 UT on 2025-04-21, i.e., 8.64 min after the EP/WXT trigger, and a series of frames were obtained in the Sloan r-bands.\n\nAn uncatalogued optical source is detected within the EP/WXT error circle at coordinates\n\nR.A.(J2000) = 11:29:40.80\nDec.(J2000) = -24:40:31.84\n\nwith an uncertainty of ~0.6 arcsec. The source has r ~ 20.6 mag, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We think the source is likely the optical counterpart of EP250421a.\n\nWe acknowledge the excellent support from T.Q. Chen and Z.K. Feng for enabling these observations.",
"published": "2025-05-10T16:09:52.969459Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.969476Z",
"modified": "2025-05-10T16:09:52.972377Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186496,
"topic": "gcn.circulars",
"uuid": "3b484a34-6ab8-474c-a8d5-a8a2874836fc",
"title": "EP250421a: JinShan likely optical counterpart",
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"authors": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40193"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: JinShan likely optical counterpart",
"createdOn": 1745261691092,
"submitter": "Dong Xu at NAOC/CAS <dxu@nao.cas.cn>",
"circularId": 40193,
"submittedHow": "web"
},
"message_text": "S.Y. Fu (HUST), S.Q. Jiang, J. An, Z.P. Zhu, X. Liu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:\n\nWe observed the field of EP250421a (ID: 01709135179) detected by EP/WXT, using the 100cm-C telescope (100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 16:25:07.32 UT on 2025-04-21, i.e., 8.64 min after the EP/WXT trigger, and a series of frames were obtained in the Sloan r-bands.\n\nAn uncatalogued optical source is detected within the EP/WXT error circle at coordinates\n\nR.A.(J2000) = 11:29:40.80\nDec.(J2000) = -24:40:31.84\n\nwith an uncertainty of ~0.6 arcsec. The source has r ~ 20.6 mag, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction. We think the source is likely the optical counterpart of EP250421a.\n\nWe acknowledge the excellent support from T.Q. Chen and Z.K. Feng for enabling these observations.",
"published": "2025-05-10T16:09:51.686307Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.686325Z",
"modified": "2025-05-10T16:09:51.689738Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186648,
"topic": "gcn.circulars",
"uuid": "b2aeec77-2b3b-4bca-a622-577cfdf2ad5a",
"title": "EP250421a: Mephisto optical counterpart confirmation",
"submitter": "liuxk@ynu.edu.cn",
"authors": "liuxk@ynu.edu.cn",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40197"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40197....1L",
"eventId": "EP250421a",
"subject": "EP250421a: Mephisto optical counterpart confirmation",
"createdOn": 1745296133708,
"submitter": "liuxk@ynu.edu.cn",
"circularId": 40197,
"submittedHow": "web"
},
"message_text": "Xiangkun Liu, Xinlei Chen, Yi Zhang, Runnan Jiang, Brajesh Kumar, Guowang Du, Yuan Fang, Jinghua Zhang, Xingzhu Zou, Yu Pan, Yuanpei Yang, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:\n\nThe field of EP250421a (EP-WXT trigger ID: 01709135179) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 16:26:55 2025-04-21 UT (~5 mins after the trigger) and 7 frames with 45s exposure time for ugi bands and 6 frames with 45s exposure times for vrz bands were taken, followed by another 2 frames with 300s exposure time for all ugi/vrz bands. We detected an optical transient candidate (Lee et al., GCN 40192; Fu et al., GCN 40193) in the stacked riz band images in both sets of observations but no detection in uvg bands. In our priliminary photometry there is clear indication of the decay of the source as listed below. \n\nStart_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)\n---------------------------|--------|---------|------------\n2025-04-21T16:26:55 | u | 45*7 | >21.15\n2025-04-21T16:38:05 | v | 45*6 | >21.39\n2025-04-21T16:26:55 | g | 45*7 | >22.02\n2025-04-21T16:38:05 | r | 45*6 | 20.47 +/- 0.10\n2025-04-21T16:26:55 | i | 45*7 | 19.07 +/- 0.07\n2025-04-21T16:38:05 | z | 45*6 | 18.62 +/- 0.13\n\nStart_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)\n---------------------------|--------|---------|------------\n2025-04-21T17:12:15 | u | 300*2 | >21.41\n2025-04-21T16:59:53 | v | 300*2 | >21.64\n2025-04-21T17:12:15 | g | 300*2 | >22.27\n2025-04-21T16:59:53 | r | 300*2 | 21.21 +/- 0.16\n2025-04-21T17:12:15 | i | 300*2 | 20.28 +/- 0.12\n2025-04-21T16:59:53 | z | 300*2 | 19.15 +/- 0.14\n\n-------------------------------------------------------------------------------------------------\nMephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.\n-------------------------------------------------------------------------------------------------\n",
"published": "2025-05-10T16:09:52.977390Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.977407Z",
"modified": "2025-05-10T16:09:52.980533Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186500,
"topic": "gcn.circulars",
"uuid": "c8ee2454-7b29-43fa-8ff5-cc01fcbc9872",
"title": "EP250421a: Mephisto optical counterpart confirmation",
"submitter": "liuxk@ynu.edu.cn",
"authors": "liuxk@ynu.edu.cn",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40197"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Mephisto optical counterpart confirmation",
"createdOn": 1745296133708,
"submitter": "liuxk@ynu.edu.cn",
"circularId": 40197,
"submittedHow": "web"
},
"message_text": "Xiangkun Liu, Xinlei Chen, Yi Zhang, Runnan Jiang, Brajesh Kumar, Guowang Du, Yuan Fang, Jinghua Zhang, Xingzhu Zou, Yu Pan, Yuanpei Yang, Xiaowei Liu (all SWIFAR, YNU) report on behalf of the Mephisto Team:\n\nThe field of EP250421a (EP-WXT trigger ID: 01709135179) was observed with the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) of Yunnan University located at Lijiang Observatory. Simultaneous uvgriz band photometric observations were conducted starting from 16:26:55 2025-04-21 UT (~5 mins after the trigger) and 7 frames with 45s exposure time for ugi bands and 6 frames with 45s exposure times for vrz bands were taken, followed by another 2 frames with 300s exposure time for all ugi/vrz bands. We detected an optical transient candidate (Lee et al., GCN 40192; Fu et al., GCN 40193) in the stacked riz band images in both sets of observations but no detection in uvg bands. In our priliminary photometry there is clear indication of the decay of the source as listed below. \n\nStart_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)\n---------------------------|--------|---------|------------\n2025-04-21T16:26:55 | u | 45*7 | >21.15\n2025-04-21T16:38:05 | v | 45*6 | >21.39\n2025-04-21T16:26:55 | g | 45*7 | >22.02\n2025-04-21T16:38:05 | r | 45*6 | 20.47 +/- 0.10\n2025-04-21T16:26:55 | i | 45*7 | 19.07 +/- 0.07\n2025-04-21T16:38:05 | z | 45*6 | 18.62 +/- 0.13\n\nStart_Time(UT) | Band | Exp(s) | Mag/LimMag (AB)\n---------------------------|--------|---------|------------\n2025-04-21T17:12:15 | u | 300*2 | >21.41\n2025-04-21T16:59:53 | v | 300*2 | >21.64\n2025-04-21T17:12:15 | g | 300*2 | >22.27\n2025-04-21T16:59:53 | r | 300*2 | 21.21 +/- 0.16\n2025-04-21T17:12:15 | i | 300*2 | 20.28 +/- 0.12\n2025-04-21T16:59:53 | z | 300*2 | 19.15 +/- 0.14\n\n-------------------------------------------------------------------------------------------------\nMephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels was achieved on 2023 December 21.\n-------------------------------------------------------------------------------------------------\n",
"published": "2025-05-10T16:09:51.720694Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.720712Z",
"modified": "2025-05-10T16:09:51.724048Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186615,
"topic": "gcn.circulars",
"uuid": "bc5ffe6a-7b41-4fe9-8d34-302829ef0896",
"title": "EP250421a: Nickel optical upper limit",
"submitter": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"authors": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40220"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40220....1M",
"eventId": "EP250421a",
"subject": "EP250421a: Nickel optical upper limit",
"createdOn": 1745450016545,
"submitter": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"circularId": 40220,
"submittedHow": "email"
},
"message_text": "Katherine Mora, Eyouel Abate, Ansel Parke, WeiKang Zheng\n\nand Alex Filippenko (UC Berkeley) report on behalf of the\n\nKAIT GRB team:\n\n\nWe observed the field of EP250421a (Zhao et al., GCN 40198)\n\nwith the 1-m Nickel telescope located at Lick observatory,\n\nCalifornia. Observations were performed in the R band with\n\n600s x 6 exposures. In our coadd image, we do not detect the\n\noptical afterglow (Lee et al., GCN. 40192; Fu et al., GCN\n\n40193; Liu et al., GCN 40197), with an upper limit of R >\n\n20.5 (Vega) at a mid-time of 14.9 hours after the trigger.\n",
"published": "2025-05-10T16:09:52.687350Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.687367Z",
"modified": "2025-05-10T16:09:52.690296Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186525,
"topic": "gcn.circulars",
"uuid": "bd963f79-8456-48eb-bae2-fc558df1a53a",
"title": "EP250421a: Nickel optical upper limit",
"submitter": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"authors": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40220"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Nickel optical upper limit",
"createdOn": 1745450016545,
"submitter": "Weikang Zheng at UC Berkeley <weikang@berkeley.edu>",
"circularId": 40220,
"submittedHow": "email"
},
"message_text": "Katherine Mora, Eyouel Abate, Ansel Parke, WeiKang Zheng\n\nand Alex Filippenko (UC Berkeley) report on behalf of the\n\nKAIT GRB team:\n\n\nWe observed the field of EP250421a (Zhao et al., GCN 40198)\n\nwith the 1-m Nickel telescope located at Lick observatory,\n\nCalifornia. Observations were performed in the R band with\n\n600s x 6 exposures. In our coadd image, we do not detect the\n\noptical afterglow (Lee et al., GCN. 40192; Fu et al., GCN\n\n40193; Liu et al., GCN 40197), with an upper limit of R >\n\n20.5 (Vega) at a mid-time of 14.9 hours after the trigger.\n",
"published": "2025-05-10T16:09:51.927958Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.927975Z",
"modified": "2025-05-10T16:09:51.930934Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186663,
"topic": "gcn.circulars",
"uuid": "f78fa30e-f8fe-426d-9b8b-96e7edde9aa2",
"title": "EP250421a: NOT optical observations",
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"authors": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40207"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40207....1M",
"eventId": "EP250421a",
"subject": "EP250421a: NOT optical observations",
"createdOn": 1745329569541,
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"circularId": 40207,
"submittedHow": "web"
},
"message_text": "D. B. Malesani (DAWN/NBI and Radboud), J. Sanchez-Sierras (Radboud), A. Martin-Carrillo (UCD), J. Quirola-Vásquez (Radboud), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), Z. P. Zhu (NAOC), H. Dawson (IAP Potsdam), report on behalf of a larger collaboration:\n\nWe observed the field of the fast X-ray transient EP250421a (Zhao et al., GCN 40198) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out under poor seeing (1.8\") in the r and z filters (3x300 s, each) starting on 2025-04-21 at 22:04:12 UT (5.79 hr after the trigger).\n\nAt the location of the optical counterpart reported by Lee et al. (GCN 40192), Fu et al. (GCN 40193), and Liu et al. (GCN 40197), also consistent with the EP/FXT and Swift/XRT positions (Zhao et al., GCN 40198; Salvaggio et al., GCN 40204), we only marginally detect a source with r = 23.8 +/- 0.3 (AB), calibrated against nearby Pan-STARRS objects.\n",
"published": "2025-05-10T16:09:53.104909Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.104926Z",
"modified": "2025-05-10T16:09:53.108178Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186510,
"topic": "gcn.circulars",
"uuid": "dd160e50-db34-45c1-b1f2-44622da62df1",
"title": "EP250421a: NOT optical observations",
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"authors": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40207"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: NOT optical observations",
"createdOn": 1745329569541,
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"circularId": 40207,
"submittedHow": "web"
},
"message_text": "D. B. Malesani (DAWN/NBI and Radboud), J. Sanchez-Sierras (Radboud), A. Martin-Carrillo (UCD), J. Quirola-Vásquez (Radboud), P. G. Jonker (Radboud), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), Z. P. Zhu (NAOC), H. Dawson (IAP Potsdam), report on behalf of a larger collaboration:\n\nWe observed the field of the fast X-ray transient EP250421a (Zhao et al., GCN 40198) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were carried out under poor seeing (1.8\") in the r and z filters (3x300 s, each) starting on 2025-04-21 at 22:04:12 UT (5.79 hr after the trigger).\n\nAt the location of the optical counterpart reported by Lee et al. (GCN 40192), Fu et al. (GCN 40193), and Liu et al. (GCN 40197), also consistent with the EP/FXT and Swift/XRT positions (Zhao et al., GCN 40198; Salvaggio et al., GCN 40204), we only marginally detect a source with r = 23.8 +/- 0.3 (AB), calibrated against nearby Pan-STARRS objects.\n",
"published": "2025-05-10T16:09:51.801926Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.801943Z",
"modified": "2025-05-10T16:09:51.805102Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186508,
"topic": "gcn.circulars",
"uuid": "a4b03751-9072-4f89-90ab-c3e06a4ab01b",
"title": "EP250421a: REM optical upper limit",
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"authors": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40205"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: REM optical upper limit",
"createdOn": 1745313766691,
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"circularId": 40205,
"submittedHow": "web"
},
"message_text": "R. Brivio, M. Ferro, D. Fugazza, P. D'Avanzo, S. Covino (INAF-OAB) report on behalf of the REM team:\n\nWe observed the field of EP250421a (Zhao et al., GCN 40198) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, and z bands, started on 2025 April 21 at 23:08:27 UT (i.e. 6.9 hr after the burst), and lasted for about 1 hour.\n\nFrom preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (Lee et al., GCN. 40192; Fu et al., GCN 40193; Liu et al., GCN 40197) down to the following 3sigma limit:\n\nr > 20.6 (AB; calibrated against the Pan-STARRS catalogue),\nat a mid-time of 7.4 hours after the trigger.",
"published": "2025-05-10T16:09:51.785648Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.785666Z",
"modified": "2025-05-10T16:09:51.788444Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186630,
"topic": "gcn.circulars",
"uuid": "3b8d67e1-a9eb-4125-a734-c18360467533",
"title": "EP250421a: REM optical upper limit",
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"authors": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40205"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40205....1B",
"eventId": "EP250421a",
"subject": "EP250421a: REM optical upper limit",
"createdOn": 1745313766691,
"submitter": "Riccardo Brivio at INAF-OAB <riccardo.brivio@inaf.it>",
"circularId": 40205,
"submittedHow": "web"
},
"message_text": "R. Brivio, M. Ferro, D. Fugazza, P. D'Avanzo, S. Covino (INAF-OAB) report on behalf of the REM team:\n\nWe observed the field of EP250421a (Zhao et al., GCN 40198) with the REM 60 cm robotic telescope located at the ESO observatory of La Silla (Chile). The observations were carried in the g, r, i, and z bands, started on 2025 April 21 at 23:08:27 UT (i.e. 6.9 hr after the burst), and lasted for about 1 hour.\n\nFrom preliminary inspection, we do not detect any counterpart at the position of the optical afterglow (Lee et al., GCN. 40192; Fu et al., GCN 40193; Liu et al., GCN 40197) down to the following 3sigma limit:\n\nr > 20.6 (AB; calibrated against the Pan-STARRS catalogue),\nat a mid-time of 7.4 hours after the trigger.",
"published": "2025-05-10T16:09:52.811946Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.811971Z",
"modified": "2025-05-10T16:09:52.815545Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186629,
"topic": "gcn.circulars",
"uuid": "7e59619c-3f86-4590-a70a-8bf20a0659a0",
"title": "EP250421a: Swift/XRT counterpart detection",
"submitter": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"authors": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40204"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40204....1S",
"eventId": "EP250421a",
"subject": "EP250421a: Swift/XRT counterpart detection",
"createdOn": 1745312671022,
"submitter": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"circularId": 40204,
"submittedHow": "email"
},
"message_text": "C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), M.\nFerro (INAF-OAB), K. Page (U. Leicester), D. Malesani (DAWN/NBI and\nRadboud) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:\n\nSwift/XRT has performed follow-up observations of EP-WXT detected transient\nEP250421a (trigger 01709135179, Zhao et al. GCN 40198).\nWe searched for X-ray sources in 1.8 ks of Photon Counting (PC) mode data,\nfrom T0+5.8 ks to T0+11.7 ks after the EP-WXT trigger.\nWe found an uncatalogued X-ray source within the estimated 3-sigma EP-WXT\nerror region. Using 1772 s of XRT PC mode data and 2 UVOT images, we find\nan enhanced XRT position: RA, Dec = 172.420,-24.67568 which is equivalent\nto:\n\nRA (J2000): 11h 29m 40.80s\nDec (J2000): -24° 40' 32.4\"\n\nwith an uncertainty of 2.7 arcsec (radius, 90% confidence).\nThis position is 35 arcsec from the EP-WXT position, and consistent with\nthe X-ray counterpart found in EP/FXT data (Zhao et al., GCN 40198) and\nwith the optical counterpart reported by Lee et al. (GCN 40192), Fu et al.\n(GCN 40193), Liu et al. (GCN 40197).\n\nThe source has a mean count rate of 0.272 ct/s and shows signs of fading at\n2.8 sigma significance.\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/EP/EP_FIELD00035/\n\nThis circular is an official product of the Swift-XRT team.\n",
"published": "2025-05-10T16:09:52.803118Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.803135Z",
"modified": "2025-05-10T16:09:52.806192Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186507,
"topic": "gcn.circulars",
"uuid": "40bd9981-7c14-44e1-ba6f-630afda50dca",
"title": "EP250421a: Swift/XRT counterpart detection",
"submitter": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"authors": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40204"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Swift/XRT counterpart detection",
"createdOn": 1745312671022,
"submitter": "Chiara Salvaggio at INAF OABrera <chiara.salvaggio@inaf.it>",
"circularId": 40204,
"submittedHow": "email"
},
"message_text": "C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), M.\nFerro (INAF-OAB), K. Page (U. Leicester), D. Malesani (DAWN/NBI and\nRadboud) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team:\n\nSwift/XRT has performed follow-up observations of EP-WXT detected transient\nEP250421a (trigger 01709135179, Zhao et al. GCN 40198).\nWe searched for X-ray sources in 1.8 ks of Photon Counting (PC) mode data,\nfrom T0+5.8 ks to T0+11.7 ks after the EP-WXT trigger.\nWe found an uncatalogued X-ray source within the estimated 3-sigma EP-WXT\nerror region. Using 1772 s of XRT PC mode data and 2 UVOT images, we find\nan enhanced XRT position: RA, Dec = 172.420,-24.67568 which is equivalent\nto:\n\nRA (J2000): 11h 29m 40.80s\nDec (J2000): -24° 40' 32.4\"\n\nwith an uncertainty of 2.7 arcsec (radius, 90% confidence).\nThis position is 35 arcsec from the EP-WXT position, and consistent with\nthe X-ray counterpart found in EP/FXT data (Zhao et al., GCN 40198) and\nwith the optical counterpart reported by Lee et al. (GCN 40192), Fu et al.\n(GCN 40193), Liu et al. (GCN 40197).\n\nThe source has a mean count rate of 0.272 ct/s and shows signs of fading at\n2.8 sigma significance.\n\nThe results of the XRT-team automatic analysis are available at\nhttps://www.swift.ac.uk/EP/EP_FIELD00035/\n\nThis circular is an official product of the Swift-XRT team.\n",
"published": "2025-05-10T16:09:51.777779Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.777797Z",
"modified": "2025-05-10T16:09:51.780891Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186523,
"topic": "gcn.circulars",
"uuid": "908691c6-e5ce-46f7-9a4a-d9886466e4a0",
"title": "EP250421a: Upper limits from Fermi-GBM Observations",
"submitter": "mariaedvige.ravasio@ru.nl",
"authors": "mariaedvige.ravasio@ru.nl",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40218"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250421a",
"subject": "EP250421a: Upper limits from Fermi-GBM Observations",
"createdOn": 1745418385761,
"submitter": "mariaedvige.ravasio@ru.nl",
"circularId": 40218,
"submittedHow": "web"
},
"message_text": "M. E. Ravasio (Radboud Univ.), E. Burns (LSU), C. Malacaria (INAF-OAR) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:\n\nFermi-GBM had full spatial coverage of the transient EP250421a detected by EP-WXT (Zhao et al., GCN 40198). There was no Fermi-GBM onboard trigger around the EP starting time T0=2025-04-21T16:16:28 UTC.\n\nThe GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50;T0+500] s, seeking signals between 64 ms and 32.768 s in duration. A candidate was found at T0+380 s, but its location is not consistent with the EP transient. No signal consistent with the EP transient, both temporally and spatially, is identified, as confirmed also by visual inspection of the data. \n\nAssuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a flux upper limit of 3.7e-08 erg/cm2/s in the energy band 10-1000 keV.\n\n[1] Goldstein et al. 2019 arXiv:1903.12597\n",
"published": "2025-05-10T16:09:51.912035Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:51.912052Z",
"modified": "2025-05-10T16:09:51.915082Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186650,
"topic": "gcn.circulars",
"uuid": "5e29760d-c157-4c11-aa2a-7d7c728016b9",
"title": "EP250421a: Upper limits from Fermi-GBM Observations",
"submitter": "mariaedvige.ravasio@ru.nl",
"authors": "mariaedvige.ravasio@ru.nl",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40218"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40218....1R",
"eventId": "EP250421a",
"subject": "EP250421a: Upper limits from Fermi-GBM Observations",
"createdOn": 1745418385761,
"submitter": "mariaedvige.ravasio@ru.nl",
"circularId": 40218,
"submittedHow": "web"
},
"message_text": "M. E. Ravasio (Radboud Univ.), E. Burns (LSU), C. Malacaria (INAF-OAR) and P.G. Jonker (Radboud Univ.) report on behalf of the Fermi-GBM Team:\n\nFermi-GBM had full spatial coverage of the transient EP250421a detected by EP-WXT (Zhao et al., GCN 40198). There was no Fermi-GBM onboard trigger around the EP starting time T0=2025-04-21T16:16:28 UTC.\n\nThe GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [T0-50;T0+500] s, seeking signals between 64 ms and 32.768 s in duration. A candidate was found at T0+380 s, but its location is not consistent with the EP transient. No signal consistent with the EP transient, both temporally and spatially, is identified, as confirmed also by visual inspection of the data. \n\nAssuming a “soft” spectral template (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), and a duration of 8.192 s, we derive a flux upper limit of 3.7e-08 erg/cm2/s in the energy band 10-1000 keV.\n\n[1] Goldstein et al. 2019 arXiv:1903.12597\n",
"published": "2025-05-10T16:09:52.993487Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.993504Z",
"modified": "2025-05-10T16:09:52.997091Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186578,
"topic": "gcn.circulars",
"uuid": "7a2dc9f9-f21b-486d-a08b-ef299e608b65",
"title": "EP 250427a: BOOTES-7 optical afterglow candidate",
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"authors": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40259"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP 250427a: BOOTES-7 optical afterglow candidate",
"createdOn": 1745748674744,
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"circularId": 40259,
"submittedHow": "web"
},
"message_text": "I. Perez-Garcia, A. J. Castro-Tirado, E. Fernandez-Garcia, S.-Y. Wu, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy (IAA-CSIC), C. Perez del Pulgar (Univ. de Malaga), G. Garcia-Segura (Inst. de Astronomia, UNAM), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS), Y.-D. Hu (GuangXi Univ.), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:\n\nFollowing the detection of EP 250427a by EP (Wang et al., GCNC 40257), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) automatically responded to this fast X-ray transient starting on Apr 27, 07:21:09 UT (i.e., ~4 hours after detection). In the first 60 s exposure image, an uncatalogued source is detected at the EP-FXT position, at coordinates (J2000): RA = 18:29:05.7, Dec = +07:33:46.5, with a preliminary magnitude of 18.1 +/- 0.08 mag (clear filter) using GaiaDR3 Gmag as a reference, which we propose to be the optical afterglow to EP 250427a. Spectroscopic observations are encouraged.\n\nWe thank the staff at San Pedro de Atacama Celestial Observations for their excellent support.",
"published": "2025-05-10T16:09:52.378683Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.378702Z",
"modified": "2025-05-10T16:09:52.381792Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194759,
"topic": "gcn.circulars",
"uuid": "41e0ec3b-1d23-426f-b91e-917b2d9a0b98",
"title": "EP 250427a: BOOTES-7 optical afterglow candidate",
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"authors": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40259"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40259....1P",
"eventId": "EP250427a",
"subject": "EP 250427a: BOOTES-7 optical afterglow candidate",
"createdOn": 1745748674744,
"submitter": "I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>",
"circularId": 40259,
"submittedHow": "web"
},
"message_text": "I. Perez-Garcia, A. J. Castro-Tirado, E. Fernandez-Garcia, S.-Y. Wu, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy (IAA-CSIC), C. Perez del Pulgar (Univ. de Malaga), G. Garcia-Segura (Inst. de Astronomia, UNAM), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS), Y.-D. Hu (GuangXi Univ.), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:\n\nFollowing the detection of EP 250427a by EP (Wang et al., GCNC 40257), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) automatically responded to this fast X-ray transient starting on Apr 27, 07:21:09 UT (i.e., ~4 hours after detection). In the first 60 s exposure image, an uncatalogued source is detected at the EP-FXT position, at coordinates (J2000): RA = 18:29:05.7, Dec = +07:33:46.5, with a preliminary magnitude of 18.1 +/- 0.08 mag (clear filter) using GaiaDR3 Gmag as a reference, which we propose to be the optical afterglow to EP 250427a. Spectroscopic observations are encouraged.\n\nWe thank the staff at San Pedro de Atacama Celestial Observations for their excellent support.",
"published": "2025-05-12T08:09:10.699733Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:10.699750Z",
"modified": "2025-05-12T08:09:10.704045Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194877,
"topic": "gcn.circulars",
"uuid": "3b225f1e-2d35-4d73-ba81-7b86401b385b",
"title": "EP250427A: COLIBRÍ Continuing Optical Observations",
"submitter": "Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>",
"authors": "Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40271"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40271....1M",
"eventId": "EP250427A",
"subject": "EP250427A: COLIBRÍ Continuing Optical Observations",
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"submitter": "Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>",
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"message_text": "Francesco Magnani (CPPM), Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM)report:\n\nWe continued our observational campaign of EP250427a (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-28 UTC.\n\nWe observed from 2025-04-28 09:16 to 2025-04-28 09:26 UTC (29.6 to 29.8 hours after the trigger) and obtained 3 stacks in filter r, i, and g, of 3 minutes each. Our observations were performed under regular weather conditions. The data were co-added with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nIn the optical position (Perez-Garcia et al., GCN Circ. 40259; Liu et al., GCN Circ. 40260; Becerra et al., GCN Circ. 40261; Brivio et al., GCN Circ. 40264; Chornock et al., GCN Circ. 40265; Saccardi et al., GCN Circ. 40266; Pérez-Fournon et al., GCN Circ. 40267; and Xi et al., GCN Circ. 40269), we measured:\n\nr = 20.55 +/- 0.05\n\nCompared to our first epoch (Becerra et al., GCN Circ. 40261), we estimate the fading nature of the OT with a temporal index ~1.2, consistent with the value reported by Swain et al. (GCN Circ. 40270).\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-12T08:09:29.022399Z",
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{
"id": 186670,
"topic": "gcn.circulars",
"uuid": "26bd268f-5993-44cc-9488-e949cceb4ea2",
"title": "EP250427A: COLIBRÍ Continuing Optical Observations",
"submitter": "Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>",
"authors": "Francesco Magnani at Aix-Marseille Université, CPPM/CNRS <francesco.magnani.work@gmail.com>",
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"message_text": "Francesco Magnani (CPPM), Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM)report:\n\nWe continued our observational campaign of EP250427a (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-28 UTC.\n\nWe observed from 2025-04-28 09:16 to 2025-04-28 09:26 UTC (29.6 to 29.8 hours after the trigger) and obtained 3 stacks in filter r, i, and g, of 3 minutes each. Our observations were performed under regular weather conditions. The data were co-added with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nIn the optical position (Perez-Garcia et al., GCN Circ. 40259; Liu et al., GCN Circ. 40260; Becerra et al., GCN Circ. 40261; Brivio et al., GCN Circ. 40264; Chornock et al., GCN Circ. 40265; Saccardi et al., GCN Circ. 40266; Pérez-Fournon et al., GCN Circ. 40267; and Xi et al., GCN Circ. 40269), we measured:\n\nr = 20.55 +/- 0.05\n\nCompared to our first epoch (Becerra et al., GCN Circ. 40261), we estimate the fading nature of the OT with a temporal index ~1.2, consistent with the value reported by Swain et al. (GCN Circ. 40270).\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-10T16:09:53.169779Z",
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{
"id": 186674,
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"uuid": "e715d301-402c-4aaf-80e9-aaee897bb43d",
"title": "EP250427A: COLIBRÍ Optical Detection",
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"authors": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
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"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
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"message_text": "Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Jean-Luc Atteia (IRAP), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA/IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM) report:\n\nWe imaged the field of EP250427A (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-27 UTC.\n\nWe observed from 2025-04-27 08:54 to 09:27 UTC (T+5.3 to T+5.8 hours after the trigger) and obtained 3-minute exposures in the g, r, and i filters (for each one). Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nWithin the EP/FXT (Wang et al., GCN Circ. 40257) error region, we detect an uncatalogued source at R.A. = 277.27390 deg, DEC = 7.56287 deg (J2000), with an uncertainty of 0.4 arcsec, with magnitudes of:\n\ng= 18.91 +/- 0.04\nr= 18.42 +/- 0.04\ni= 17.88 +/- 0.04\n\nThis source is consistent with the candidate reported by BOOTES-7 (Perez-Garcia et al., GCN Circ. 40259) and TRT (Liu et al., GCN Circ. 40260).\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-10T16:09:53.201104Z",
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{
"id": 186580,
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"uuid": "32c0c8c3-f2d2-4d43-8f69-4945ac433cf9",
"title": "EP250427A: COLIBRÍ Optical Detection",
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"authors": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
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"message_text": "Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Jean-Luc Atteia (IRAP), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA/IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM) report:\n\nWe imaged the field of EP250427A (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-27 UTC.\n\nWe observed from 2025-04-27 08:54 to 09:27 UTC (T+5.3 to T+5.8 hours after the trigger) and obtained 3-minute exposures in the g, r, and i filters (for each one). Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nWithin the EP/FXT (Wang et al., GCN Circ. 40257) error region, we detect an uncatalogued source at R.A. = 277.2746 deg, DEC = 7.5639 deg (J2000), with an uncertainty of 0.4 arcsec, with magnitudes of:\n\ng= 18.91 +/- 0.04\nr= 18.42 +/- 0.04\ni= 17.88 +/- 0.04\n\nThis source is consistent with the candidate reported by BOOTES-7 (Perez-Garcia et al., GCN Circ. 40259) and TRT (Liu et al., GCN Circ. 40260).\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-10T16:09:52.395640Z",
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{
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"uuid": "1e5565ac-4309-438e-a4d0-10b2fc782f3c",
"title": "EP250427A: COLIBRÍ Optical Detection",
"submitter": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
"authors": "Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
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"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>",
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"message_text": "Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Jean-Luc Atteia (IRAP), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA/IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM) and Antonio de Ugarte Postigo (LAM) report:\n\nWe imaged the field of EP250427A (Wang et al., GCN Circ. 40257) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico on the night of 2025-04-27 UTC.\n\nWe observed from 2025-04-27 08:54 to 09:27 UTC (T+5.3 to T+5.8 hours after the trigger) and obtained 3-minute exposures in the g, r, and i filters (for each one). Our observations were performed under regular weather conditions. The data were coadded with the COLIBRÍ pipeline and analysed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.\n\nWithin the EP/FXT (Wang et al., GCN Circ. 40257) error region, we detect an uncatalogued source at R.A. = 277.27390 deg, DEC = 7.56287 deg (J2000), with an uncertainty of 0.4 arcsec, with magnitudes of:\n\ng= 18.91 +/- 0.04\nr= 18.42 +/- 0.04\ni= 17.88 +/- 0.04\n\nThis source is consistent with the candidate reported by BOOTES-7 (Perez-Garcia et al., GCN Circ. 40259) and TRT (Liu et al., GCN Circ. 40260).\n\nFurther observations and analysis are ongoing.\n\nWe warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.\n",
"published": "2025-05-12T08:09:27.582925Z",
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{
"id": 186576,
"topic": "gcn.circulars",
"uuid": "5d8e46fe-02a0-4a85-b84a-53c22128a8d1",
"title": "EP250427a: Einstein Probe detection of a fast X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
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"eventId": "EP250427a",
"subject": "EP250427a: Einstein Probe detection of a fast X-ray transient",
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"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
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"message_text": "Y.Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y, Liu (NAO, CAS) behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250427a (GCN Notice ID 01709135324). The WXT position of the source is R.A. = 277.281 deg, DEC = 7.570 deg (J2000) with an uncertainty of 2.0 arcmin in radius (90% C.L. statistical and systematic). \nA preliminary analysis of the EP-WXT data shows that the transient began at 2025-04-27T03:38:45(UTC) (before which the satellite was in the SAA region, and the real start time might be earlier than the value reported here) and lasted for about 180s, with a peak flux of 2 x 10^-8 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a photon index of 1.70 (+0.36, -0.34) and a column density of 3.92 (+0.14, -0.13) x 10^21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.96 (+0.31, -0.23) x 10^-9 erg/cm^2/s.\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed about 5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 277.2746 deg, DEC = 7.5639 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received.\n\nThe contact TA of EP250427a is Y. Wang. Please contact him via email wangyun@pmo.ac.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:52.361330Z",
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{
"id": 186679,
"topic": "gcn.circulars",
"uuid": "aa34d94a-d9ea-4ff7-b622-a7bccdacf308",
"title": "EP250427a: Einstein Probe detection of a fast X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
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"eventId": "EP250427a",
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"version": 2,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
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"message_text": "Y. Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y. Liu (NAO, CAS) behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250427a (GCN Notice ID 01709135324). The WXT position of the source is R.A. = 277.281 deg, DEC = 7.570 deg (J2000) with an uncertainty of 2.0 arcmin in radius (90% C.L. statistical and systematic). \nA preliminary analysis of the EP-WXT data shows that the transient began at 2025-04-27T03:38:45(UTC) (before which the satellite was in the SAA region, and the real start time might be earlier than the value reported here) and lasted for about 180s, with a peak flux of 2 x 10^-8 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a photon index of 1.70 (+0.36, -0.34) and a column density of 3.92 (+0.14, -0.13) x 10^21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.96 (+0.31, -0.23) x 10^-9 erg/cm^2/s.\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed about 5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 277.2746 deg, DEC = 7.5639 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received.\n\nThe contact TA of EP250427a is Y. Wang. Please contact him via email wangyun@pmo.ac.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-10T16:09:53.243077Z",
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{
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"topic": "gcn.circulars",
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"title": "EP250427a: Einstein Probe detection of a fast X-ray transient",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
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"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
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"submittedHow": "web"
},
"message_text": "Y. Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y. Liu (NAO, CAS) behalf of the Einstein Probe (EP) team:\n\nWe report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP250427a (GCN Notice ID 01709135324). The WXT position of the source is R.A. = 277.281 deg, DEC = 7.570 deg (J2000) with an uncertainty of 2.0 arcmin in radius (90% C.L. statistical and systematic). \nA preliminary analysis of the EP-WXT data shows that the transient began at 2025-04-27T03:38:45(UTC) (before which the satellite was in the SAA region, and the real start time might be earlier than the value reported here) and lasted for about 180s, with a peak flux of 2 x 10^-8 erg/cm^2/s. The averaged WXT spectrum can be fitted by an absorbed power law model with a photon index of 1.70 (+0.36, -0.34) and a column density of 3.92 (+0.14, -0.13) x 10^21 cm^-2. The unabsorbed 0.5-4 keV flux is about 1.96 (+0.31, -0.23) x 10^-9 erg/cm^2/s.\n\nA follow-up observation with the Follow-up X-ray Telescope (FXT) was performed about 5 hours after the WXT detection. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 277.2746 deg, DEC = 7.5639 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received.\n\nThe contact TA of EP250427a is Y. Wang. Please contact him via email wangyun@pmo.ac.cn if needed.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).",
"published": "2025-05-12T08:09:28.807075Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:28.807091Z",
"modified": "2025-05-12T08:09:28.810933Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186676,
"topic": "gcn.circulars",
"uuid": "64230c17-5ed4-4e57-a08e-09a2592e031b",
"title": "EP250427a: EP-FXT follow-up observations",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40274"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a: EP-FXT follow-up observations",
"createdOn": 1745910712726,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40274,
"submittedHow": "web"
},
"message_text": "Y. Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y. Liu (NAO, CAS) behalf of the Einstein Probe (EP) team:\n\nAfter the EP-WXT detection of the fast X-ray transient EP250427a (Wang et al., GCN 40257), two follow-up observations have been conducted by EP-FXT.\n\nThe first follow-up observation was performed at 2025-04-27T08:38:17 (UTC), about 5.6 hours after the trigger, with an exposure time of 4.1 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source within the EP-WXT error circle, at the coordinates (J2000): R.A., Dec. = 277.2728, 7.5625 deg, with an uncertainty of about 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). The 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the column density nH fixed at the Galactic value of 3.01e21 cm^-2, and a photon index of 1.89(+0.18, -0.18). The unabsorbed 0.5-10 keV flux is 4.2 (+0.6, -0.5) e-12 erg/s/cm^2. This is consistent with the Swift-XRT refined analysis (D'Elia et al. GCN 40268).\n\nThe FXT localization is consistant with the positions of the optical conterparts. The detection of the optical and IR counterpart has been reported in several GCNs (Lipunov et al. GCN 40258; Perez-Garcia et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272), and the event was also detected as the sub-threshold GRB 250427A by Fermi/GBM (Ravasio et al. GCN 40262). And the redshift of EP240527a was measured to be 1.519 (Chornock et al. GCN 40265) or 1.520 (Saccardi et al. GCN40266). \n\nThe second follow-up observation was performed at 2025-04-28T14:13:49 (UTC), about 35.1 hours after the trigger, with an exposure time of 5.9 ks. The 0.5-10 keV spectrum can also be fitted with an absorbed power-law model, with nH fixed at the Galactic value of 3.01e21cm^-2, and a photon index fixed of 1.66 (+0.44, -0.41). The unabsorbed 0.5-10 keV flux is 5.0 (+2.1, -1.3) e-13 erg/s/cm^2.\n\nAll uncertainties quoted above are at the 90% confidence level.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).\n\n",
"published": "2025-05-10T16:09:53.217758Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.217775Z",
"modified": "2025-05-10T16:09:53.221003Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194878,
"topic": "gcn.circulars",
"uuid": "ff5f68d3-9a38-493e-87f4-f7aeb8e7ef28",
"title": "EP250427a: EP-FXT follow-up observations",
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"authors": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40274"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40274....1W",
"eventId": "EP250427a",
"subject": "EP250427a: EP-FXT follow-up observations",
"createdOn": 1745910712726,
"submitter": "EP Team at NAOC/CAS <ep_ta@bao.ac.cn>",
"circularId": 40274,
"submittedHow": "web"
},
"message_text": "Y. Wang (PMO, CAS), D. Y. Li (NAO, CAS), Y. Q. Zhao (USTC, PRIC), J. H. Wu (GZHU), Y. Liu (NAO, CAS) behalf of the Einstein Probe (EP) team:\n\nAfter the EP-WXT detection of the fast X-ray transient EP250427a (Wang et al., GCN 40257), two follow-up observations have been conducted by EP-FXT.\n\nThe first follow-up observation was performed at 2025-04-27T08:38:17 (UTC), about 5.6 hours after the trigger, with an exposure time of 4.1 ks. On-ground analysis of the EP-FXT data identified an uncatalogued X-ray source within the EP-WXT error circle, at the coordinates (J2000): R.A., Dec. = 277.2728, 7.5625 deg, with an uncertainty of about 10 arcsec (radius, 90% confidence level, including both statistical and systematic errors). The 0.5-10 keV spectrum can be fitted with an absorbed power-law model, with the column density nH fixed at the Galactic value of 3.01e21 cm^-2, and a photon index of 1.89(+0.18, -0.18). The unabsorbed 0.5-10 keV flux is 4.2 (+0.6, -0.5) e-12 erg/s/cm^2. This is consistent with the Swift-XRT refined analysis (D'Elia et al. GCN 40268).\n\nThe FXT localization is consistant with the positions of the optical conterparts. The detection of the optical and IR counterpart has been reported in several GCNs (Lipunov et al. GCN 40258; Perez-Garcia et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272), and the event was also detected as the sub-threshold GRB 250427A by Fermi/GBM (Ravasio et al. GCN 40262). And the redshift of EP240527a was measured to be 1.519 (Chornock et al. GCN 40265) or 1.520 (Saccardi et al. GCN40266). \n\nThe second follow-up observation was performed at 2025-04-28T14:13:49 (UTC), about 35.1 hours after the trigger, with an exposure time of 5.9 ks. The 0.5-10 keV spectrum can also be fitted with an absorbed power-law model, with nH fixed at the Galactic value of 3.01e21cm^-2, and a photon index fixed of 1.66 (+0.44, -0.41). The unabsorbed 0.5-10 keV flux is 5.0 (+2.1, -1.3) e-13 erg/s/cm^2.\n\nAll uncertainties quoted above are at the 90% confidence level.\n\nLaunched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).\n\n",
"published": "2025-05-12T08:09:29.031689Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:29.031706Z",
"modified": "2025-05-12T08:09:29.035577Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194819,
"topic": "gcn.circulars",
"uuid": "7da70aeb-8294-4cc1-b3a2-674239cc5f0d",
"title": "EP250427a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40258"
},
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40258....1L",
"eventId": "EP250427a",
"subject": "EP250427a: Global MASTER-Net observations report",
"createdOn": 1745748587549,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40258,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, \nG.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA.Sosnovskij (CrAO),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250427a ( EP Team et al., GCN 40257) errorbox 254 sec after notice time and 21870 sec after trigger time at 2025-04-27 09:43:15 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 41 deg. The sun altitude is -17.5 deg. \n\nThe galactic latitude b = 8 deg., longitude l = 37 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2854486\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 21901 | 2025-04-27 09:43:15 | MASTER-OAFA | (18h 28m 05.12s , +07d 20m 04.3s) | C | 60 | 18.4 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-12T08:09:26.418215Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:26.418232Z",
"modified": "2025-05-12T08:09:26.422178Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186577,
"topic": "gcn.circulars",
"uuid": "d021dfd8-88f8-4ae5-b1ea-59edb93ba428",
"title": "EP250427a: Global MASTER-Net observations report",
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"authors": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40258"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a: Global MASTER-Net observations report",
"createdOn": 1745748587549,
"submitter": "Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>",
"circularId": 40258,
"submittedHow": "email"
},
"message_text": "V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, \nG.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),\nO.Gress, N.Budnev (ISU),\nC.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),\nA. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),\nA.Sosnovskij (CrAO),\nA. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),\nD.Buckley (SAAO),\nR.Rebolo (The Instituto de Astrofisica de Canarias),\nL.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,\nA.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) \n\nMASTER-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP250427a ( EP Team et al., GCN 40257) errorbox 254 sec after notice time and 21870 sec after trigger time at 2025-04-27 09:43:15 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 41 deg. The sun altitude is -17.5 deg. \n\nThe galactic latitude b = 8 deg., longitude l = 37 deg.\n\n\nReal time updated cover map and OT discovered available here: \nhttps://master.sai.msu.ru/site/master2/observ.php?id=2854486\n\nWe obtain a following upper limits. \n\nTmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment\n_________|_____________________|_____________________|____________________________________|_____|_______|_______|________\n\n 21901 | 2025-04-27 09:43:15 | MASTER-OAFA | (18h 28m 05.12s , +07d 20m 04.3s) | C | 60 | 18.4 | \nFilter C is a clear (unfiltred) band. \n\n\nThe observation and reduction will continue. \nThe message may be cited.\n",
"published": "2025-05-10T16:09:52.369961Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.369988Z",
"modified": "2025-05-10T16:09:52.373498Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186675,
"topic": "gcn.circulars",
"uuid": "1afcb8dc-fec6-4ac9-8bac-6a991c460f0a",
"title": "EP250427a/GRB 250427A: FTW optical and NIR observations of the counterpart ",
"submitter": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"authors": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40273"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a/GRB 250427A: FTW optical and NIR observations of the counterpart ",
"createdOn": 1745860004766,
"submitter": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"circularId": 40273,
"submittedHow": "web"
},
"message_text": "Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU) and Antonella Palmese (Carnegie Mellon U.) report:\n\nWe observed the counterpart of EP250427a (Wang et al., GCN 40257; Lipunov et al., GCN 40258; Perez-Garcia et al., GCN 40259; Liu et al., GCN 40260; Becerra et al., GCN 40261; Brivio et al., GCN 40264; Saccardi et al., GCN 40266; Pérez-Fournon et al., GCN 40267; Xi et al., GCN 40269; Swain et al., GCN 40270; Magnani et al., GCN 40271; de Wet et al., GCB 40272) which was also seen as sub-threshold GRB 250427A by Fermi/GBM (Ravasio et al., GCN 40262) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-04-28T01:48:29 UT (0.92 days after the trigger). We detect the counterpart at\n\nr = (20.17 +/- 0.03) mag\ni = (19.75 +/- 0.03) mag\nJ = (18.91 +/- 0.03) mag.\n\nThe r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.\n\nWe thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.",
"published": "2025-05-10T16:09:53.209639Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.209656Z",
"modified": "2025-05-10T16:09:53.213008Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194837,
"topic": "gcn.circulars",
"uuid": "a5cee468-c34e-4178-9ee4-ad441b9dd466",
"title": "EP250427a/GRB 250427A: FTW optical and NIR observations of the counterpart ",
"submitter": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"authors": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40273"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40273....1B",
"eventId": "EP250427a",
"subject": "EP250427a/GRB 250427A: FTW optical and NIR observations of the counterpart ",
"createdOn": 1745860004766,
"submitter": "Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>",
"circularId": 40273,
"submittedHow": "web"
},
"message_text": "Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU) and Antonella Palmese (Carnegie Mellon U.) report:\n\nWe observed the counterpart of EP250427a (Wang et al., GCN 40257; Lipunov et al., GCN 40258; Perez-Garcia et al., GCN 40259; Liu et al., GCN 40260; Becerra et al., GCN 40261; Brivio et al., GCN 40264; Saccardi et al., GCN 40266; Pérez-Fournon et al., GCN 40267; Xi et al., GCN 40269; Swain et al., GCN 40270; Magnani et al., GCN 40271; de Wet et al., GCB 40272) which was also seen as sub-threshold GRB 250427A by Fermi/GBM (Ravasio et al., GCN 40262) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 180 s starting at 2025-04-28T01:48:29 UT (0.92 days after the trigger). We detect the counterpart at\n\nr = (20.17 +/- 0.03) mag\ni = (19.75 +/- 0.03) mag\nJ = (18.91 +/- 0.03) mag.\n\nThe r and i band magnitudes are calibrated against the PS1 catalog and the J band is calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.\n\nWe thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.",
"published": "2025-05-12T08:09:28.605128Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:28.605148Z",
"modified": "2025-05-12T08:09:28.611509Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194746,
"topic": "gcn.circulars",
"uuid": "7fd69fce-fae4-4425-b10b-67ef9b4e5d56",
"title": "EP250427a/GRB250427A: HCT optical follow-up",
"submitter": "anirudhsalgundi@gmail.com",
"authors": "anirudhsalgundi@gmail.com",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40289"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40289....1E",
"eventId": "EP250427a",
"subject": "EP250427a/GRB250427A: HCT optical follow-up",
"createdOn": 1746017753549,
"submitter": "anirudhsalgundi@gmail.com",
"circularId": 40289,
"submittedHow": "web"
},
"message_text": "D. Eappachen (IIA), V. Swain (IITB), A. Salgundi (IITB), D.K. Sahu (IIA), A.P. Saikia (IITB), G. C. Anupama (IIA), V. Bhalerao (IITB), A. Balasubramanian (IIA), S. Barway (IIA), S. Bandari (IAO):\n\nWe observed the field of EP250427a (Wang et al., GCN #40257), also detected by Fermi-GBM (Ravasio et al., GCN #40262) with the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO). We obtained multiple exposures in the SDSS \nr' filter starting at 2025-04-29 22:10:49.88 UT (~2.77 days post trigger). We detected the source in our stacked image and obtained the following photometric result:\n\n\n| JD (mid) | Filter | Exposure (s) | Mag (AB) | Limiting Magnitude (AB) |\n| ----------------- | ------- | ------------------ | -------------- |--------------|\n| 2460795.433742 | r' | 5 x 240 | 21.70 +/- 0.12 | 21.9 |\n\n\nThe magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.\n\nOur result is consistent with BOOTES-7 (Perez-Garcia et al., GCN #40259), TRT (Liu et al., GCN #40260), COLIBRÍ (Becerra et al., GCN #40261), REM (Brivio et al., GCN #40264), LCO (Pérez-Fournon et al., GCN #40267), LT (Qiang Xi et al., GCN #40269), GIT (Swain et al., GCN #40270), COLIBRÍ (Magnani et al., GCN #40271), Lesedi (S. de Wet et al., GCN #40272), FTW (Busmann et al., GCN #40273), OHP/T193 (Amram et al., GCN #40275).\n\nThese observations were carried out under the ToO program HCT-2025-C1-P08. We thank the HCT staff for their support during the observations. The Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics, Bengaluru, India.",
"published": "2025-05-12T08:09:05.538795Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:05.538815Z",
"modified": "2025-05-12T08:09:05.543132Z",
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},
{
"id": 186692,
"topic": "gcn.circulars",
"uuid": "c6c01c3d-f9d4-4de4-b6d5-3deafe71267f",
"title": "EP250427a/GRB250427A: HCT optical follow-up",
"submitter": "anirudhsalgundi@gmail.com",
"authors": "anirudhsalgundi@gmail.com",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40289"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a/GRB250427A: HCT optical follow-up",
"createdOn": 1746017753549,
"submitter": "anirudhsalgundi@gmail.com",
"circularId": 40289,
"submittedHow": "web"
},
"message_text": "D. Eappachen (IIA), V. Swain (IITB), A. Salgundi (IITB), D.K. Sahu (IIA), A.P. Saikia (IITB), G. C. Anupama (IIA), V. Bhalerao (IITB), A. Balasubramanian (IIA), S. Barway (IIA), S. Bandari (IAO):\n\nWe observed the field of EP250427a (Wang et al., GCN #40257), also detected by Fermi-GBM (Ravasio et al., GCN #40262) with the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO). We obtained multiple exposures in the SDSS \nr' filter starting at 2025-04-29 22:10:49.88 UT (~2.77 days post trigger). We detected the source in our stacked image and obtained the following photometric result:\n\n\n| JD (mid) | Filter | Exposure (s) | Mag (AB) | Limiting Magnitude (AB) |\n| ----------------- | ------- | ------------------ | -------------- |--------------|\n| 2460795.433742 | r' | 5 x 240 | 21.70 +/- 0.12 | 21.9 |\n\n\nThe magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.\n\nOur result is consistent with BOOTES-7 (Perez-Garcia et al., GCN #40259), TRT (Liu et al., GCN #40260), COLIBRÍ (Becerra et al., GCN #40261), REM (Brivio et al., GCN #40264), LCO (Pérez-Fournon et al., GCN #40267), LT (Qiang Xi et al., GCN #40269), GIT (Swain et al., GCN #40270), COLIBRÍ (Magnani et al., GCN #40271), Lesedi (S. de Wet et al., GCN #40272), FTW (Busmann et al., GCN #40273), OHP/T193 (Amram et al., GCN #40275).\n\nThese observations were carried out under the ToO program HCT-2025-C1-P08. We thank the HCT staff for their support during the observations. The Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics, Bengaluru, India.",
"published": "2025-05-10T16:09:53.350815Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.350832Z",
"modified": "2025-05-10T16:09:53.353922Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194744,
"topic": "gcn.circulars",
"uuid": "29814dbf-0684-4588-92c8-13a42743e068",
"title": "EP250427A/GRB250427A: Keck/LRIS redshift",
"submitter": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"authors": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40265"
},
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40265....1C",
"eventId": "GRB 250427A",
"subject": "EP250427A/GRB250427A: Keck/LRIS redshift",
"createdOn": 1745763082913,
"submitter": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"circularId": 40265,
"submittedHow": "email"
},
"message_text": "\nR.Chornock, E. Hammerstein, X. Guo (UC Berkeley) report:\n\nWe observed the optical afterglow (GCNs 40258, 40259, 40260, 40261, 40263, 40264) of EP250427a (GCN 40257)/GRB 250427a (GCN 40262) using the Low-Resolution Imaging Spectrometer on the Keck-I telescope at a mean time of 12:36UT on 2025 Apr 27. Observations covered the range 3140-10300 Angstroms. \n\nA continuum is well detected across the full spectral range with many absorption lines present. We identify a strong doublet at observed wavelengths of 3900.3,3906.7 Angs as CIV at z=1.519 from the highest redshift system detected. There are also many lines (C II, C IV, Fe II, Mg I, Mg II) from a stronger low-redshift system at z=1.406.\n\nFurther analysis is ongoing.\n\n",
"published": "2025-05-12T08:09:03.511797Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:03.511814Z",
"modified": "2025-05-12T08:09:03.515981Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186584,
"topic": "gcn.circulars",
"uuid": "1eef1a6d-cca3-4e69-bd81-78d4955b5ef3",
"title": "EP250427A/GRB250427A: Keck/LRIS redshift",
"submitter": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"authors": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40265"
},
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "GRB 250427A",
"subject": "EP250427A/GRB250427A: Keck/LRIS redshift",
"createdOn": 1745763082913,
"submitter": "Ryan Chornock at UC Berkeley <chornock@berkeley.edu>",
"circularId": 40265,
"submittedHow": "email"
},
"message_text": "\nR.Chornock, E. Hammerstein, X. Guo (UC Berkeley) report:\n\nWe observed the optical afterglow (GCNs 40258, 40259, 40260, 40261, 40263, 40264) of EP250427a (GCN 40257)/GRB 250427a (GCN 40262) using the Low-Resolution Imaging Spectrometer on the Keck-I telescope at a mean time of 12:36UT on 2025 Apr 27. Observations covered the range 3140-10300 Angstroms. \n\nA continuum is well detected across the full spectral range with many absorption lines present. We identify a strong doublet at observed wavelengths of 3900.3,3906.7 Angs as CIV at z=1.519 from the highest redshift system detected. There are also many lines (C II, C IV, Fe II, Mg I, Mg II) from a stronger low-redshift system at z=1.406.\n\nFurther analysis is ongoing.\n\n",
"published": "2025-05-10T16:09:52.427818Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.427835Z",
"modified": "2025-05-10T16:09:52.430927Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194745,
"topic": "gcn.circulars",
"uuid": "ab425773-b7be-4b1d-aad9-9466affdfff6",
"title": " EP250427a / GRB 250427A: LCO optical counterpart detection",
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"authors": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40267"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40267....1P",
"eventId": "GRB 250427A",
"subject": " EP250427a / GRB 250427A: LCO optical counterpart detection",
"createdOn": 1745771409741,
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"circularId": 40267,
"submittedHow": "web"
},
"message_text": "I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)\n\nFollowing the detection of the fast X-ray transient EP250427a (Wang et al., GCN circ. 40257), detected also by Fermi-GBM, GRB 250427A (Ravasio et al., GCN circ. 40262), we observed the field with one of the two Las Cumbres Observatory Global Telescope network (LCOGT) 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at McDonald Observatory (Texas) in the SDSS r filter. The observation started at 2025-04-27 10:10:47 UTC, about 6.53 hours after the EP-WXT trigger. An uncatalogued source is clearly detected at the optical counterpart position first reported by Perez-Garcia et al. (GCN circ. 40259) and with other optical and near-infrared detections reported by Liu et al. (GCN circ. 40260), Becerra et al. (GCN circ. 40261), Brivio et al. (GCN circ. 40264), Chornock et al. (redshift of z = 1.519, GCN circ. 40265), and Saccardi et al. (redshift of z = 1.520, GCN circ. 40266).\n\nWe measure the following magnitude, calibrated against Pan-STARRS DR2 stars, that is not corrected for Galactic extinction:\n\nDate | UT start | mag | error | filter | exposure time (sec) | \n------------------------------------------------------------------------\n2025-04-27 10:10:47 18.55 0.10 r 180\n\nThis work makes use of observations from the Las Cumbres Observatory global telescope network \n(LCOGT observing programme IAC2025A-009, SGLF).",
"published": "2025-05-12T08:09:03.522899Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:03.522916Z",
"modified": "2025-05-12T08:09:03.527029Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186586,
"topic": "gcn.circulars",
"uuid": "dc01e5a4-11d3-4488-a79c-1c4a2217ed12",
"title": " EP250427a / GRB 250427A: LCO optical counterpart detection",
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"authors": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40267"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "GRB 250427A",
"subject": " EP250427a / GRB 250427A: LCO optical counterpart detection",
"createdOn": 1745771409741,
"submitter": "Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>",
"circularId": 40267,
"submittedHow": "web"
},
"message_text": "I. Pérez-Fournon, F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia (ULL), and A. López-Oramas (IAC and ULL)\n\nFollowing the detection of the fast X-ray transient EP250427a (Wang et al., GCN circ. 40257), detected also by Fermi-GBM, GRB 250427A (Ravasio et al., GCN circ. 40262), we observed the field with one of the two Las Cumbres Observatory Global Telescope network (LCOGT) 1-m telescopes, equipped with Sinistro cameras, located at the LCOGT node at McDonald Observatory (Texas) in the SDSS r filter. The observation started at 2025-04-27 10:10:47 UTC, about 6.53 hours after the EP-WXT trigger. An uncatalogued source is clearly detected at the optical counterpart position first reported by Perez-Garcia et al. (GCN circ. 40259) and with other optical and near-infrared detections reported by Liu et al. (GCN circ. 40260), Becerra et al. (GCN circ. 40261), Brivio et al. (GCN circ. 40264), Chornock et al. (redshift of z = 1.519, GCN circ. 40265), and Saccardi et al. (redshift of z = 1.520, GCN circ. 40266).\n\nWe measure the following magnitude, calibrated against Pan-STARRS DR2 stars, that is not corrected for Galactic extinction:\n\nDate | UT start | mag | error | filter | exposure time (sec) | \n------------------------------------------------------------------------\n2025-04-27 10:10:47 18.55 0.10 r 180\n\nThis work makes use of observations from the Las Cumbres Observatory global telescope network \n(LCOGT observing programme IAC2025A-009, SGLF).",
"published": "2025-05-10T16:09:52.443978Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:52.443995Z",
"modified": "2025-05-10T16:09:52.447193Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186707,
"topic": "gcn.circulars",
"uuid": "ba878595-5b73-40e6-bb32-bf53dedd38a2",
"title": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40288"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Christophe Adami at LAM <christophe.adami@lam.fr>",
"editedOn": 1746107117751,
"createdOn": 1746017686971,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40288,
"submittedHow": "web"
},
"message_text": "P. Amram (LAM/AMU), S. Basa (Pytheas/OHP/LAM), C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), J. T. Palmerio (CEA), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:\n\nWe re-observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 3x900sec in the r’ band starting at 02:42:04UT on 2025-04-30 (T0+71.1 h after the trigger). The optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279 is still detected at r = 21.50 +/- 0.18 mag (AB). \n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular J. Schmitt, J.C. Brunel, F. Huppert, F. Moreau, Stephane Favard, Jean Pierre Troncin, Jean Balcaen and Yoann Degot-Longhi.\n\n\n",
"published": "2025-05-10T16:09:53.473402Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.473419Z",
"modified": "2025-05-10T16:09:53.476485Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 186691,
"topic": "gcn.circulars",
"uuid": "d5017991-1bc0-4bd1-92f3-d329c6066068",
"title": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40288"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"createdOn": 1746017686971,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40288,
"submittedHow": "web"
},
"message_text": "P. Amram (LAM/AMU), S. Basa (Pytheas/OHP/LAM), C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), J. T. Palmerio (CEA), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:\n\nWe re-observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 3x900sec in the r’ band starting at 02:42:04UT on 2025-04-30 (T0+47.1 h after the trigger). The optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279 is still detected at r = 21.50 +/- 0.18 mag (AB). \n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular J. Schmitt, J.C. Brunel, F. Huppert, F. Moreau, Stephane Favard, Jean Pierre Troncin, Jean Balcaen and Yoann Degot-Longhi.\n\n\n",
"published": "2025-05-10T16:09:53.342537Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.342554Z",
"modified": "2025-05-10T16:09:53.345695Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194821,
"topic": "gcn.circulars",
"uuid": "8fb1f8f5-d779-44e8-b3d7-657d99e9e54f",
"title": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40288"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40288....1A",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 continued photometric observations",
"version": 2,
"editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Christophe Adami at LAM <christophe.adami@lam.fr>",
"editedOn": 1746107117751,
"createdOn": 1746017686971,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40288,
"submittedHow": "web"
},
"message_text": "P. Amram (LAM/AMU), S. Basa (Pytheas/OHP/LAM), C. Adami (LAM/Pytheas/AMU), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), J. T. Palmerio (CEA), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:\n\nWe re-observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 3x900sec in the r’ band starting at 02:42:04UT on 2025-04-30 (T0+71.1 h after the trigger). The optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279 is still detected at r = 21.50 +/- 0.18 mag (AB). \n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular J. Schmitt, J.C. Brunel, F. Huppert, F. Moreau, Stephane Favard, Jean Pierre Troncin, Jean Balcaen and Yoann Degot-Longhi.\n\n\n",
"published": "2025-05-12T08:09:26.439878Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:26.439894Z",
"modified": "2025-05-12T08:09:26.444131Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194861,
"topic": "gcn.circulars",
"uuid": "c0441b49-f652-4e73-8f4f-577b91c410a4",
"title": "EP250427a / GRB 250427A: OHP/T193 photometric observations",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40275"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40275....1A",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 photometric observations",
"createdOn": 1745916540951,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40275,
"submittedHow": "web"
},
"message_text": "P. Amram (LAM/AMU), C. Adami (LAM/Pytheas/AMU), S. Vergani (CNRS, Obs. de Paris, LUX), S. Basa (Pytheas/OHP/LAM), A. Saccardi (CEA/Irfu), B. Schneider (LAM) report on behalf of the MISTRAL-GRB collaboration:\n\nWe observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 2x60sec + 120sec + 3x600sec in the r’ band starting at 02:55:47UT on 2025-04-28 (T0+23.3 h after the trigger). The optical afterglow reported by Perez-Garcia et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273 is detected at a preliminar value of r = 20.3 +/- 0.15 mag (AB). \n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen and Yoann Degot-Longhi.\n",
"published": "2025-05-12T08:09:28.857776Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:28.857792Z",
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{
"id": 186677,
"topic": "gcn.circulars",
"uuid": "e5694c00-bbf3-4872-b2d4-a32a93f81c6a",
"title": "EP250427a / GRB 250427A: OHP/T193 photometric observations",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40275"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.2.0/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 photometric observations",
"createdOn": 1745916540951,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40275,
"submittedHow": "web"
},
"message_text": "P. Amram (LAM/AMU), C. Adami (LAM/Pytheas/AMU), S. Vergani (CNRS, Obs. de Paris, LUX), S. Basa (Pytheas/OHP/LAM), A. Saccardi (CEA/Irfu), B. Schneider (LAM) report on behalf of the MISTRAL-GRB collaboration:\n\nWe observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 2x60sec + 120sec + 3x600sec in the r’ band starting at 02:55:47UT on 2025-04-28 (T0+23.3 h after the trigger). The optical afterglow reported by Perez-Garcia et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273 is detected at a preliminar value of r = 20.3 +/- 0.15 mag (AB). \n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen and Yoann Degot-Longhi.\n",
"published": "2025-05-10T16:09:53.226566Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.226582Z",
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},
{
"id": 194870,
"topic": "gcn.circulars",
"uuid": "73c85059-1a04-41e5-b4e0-11097699155e",
"title": "EP250427a / GRB 250427A: OHP/T193 photometric observations: third visit",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40306"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40306....1A",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 photometric observations: third visit",
"createdOn": 1746115933889,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40306,
"submittedHow": "web"
},
"message_text": "C. Adami (LAM/Pytheas/AMU), S. Basa (Pytheas/OHP/LAM), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:\n\nWe observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) a third time using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 1x72sec + 1x600sec + 1x900sec + 4x720sec in the r’ band starting at 01:34:13UT on 2025-05-01 (T0+93.9 h after the trigger). In the stacked image, we do not significantly detect the optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279, Amram et al. GCN 40288, Eappachen et al. GCN 40289, and Ghosh et al. GCN 40293 down to the following 5-sigma upper limit: r > 22.8.\nIf we force the detection at the EP250427a afterglow position, we get a very marginal detection just below the detection limit with a preliminary magnitude of r=23.1+/-0.35\n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen, and the SOPHIE observer Owin Scutt.\n",
"published": "2025-05-12T08:09:28.949749Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:28.949766Z",
"modified": "2025-05-12T08:09:28.953606Z",
"nonlocalizedevents": [],
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},
{
"id": 186710,
"topic": "gcn.circulars",
"uuid": "34e31aec-c40d-47f2-a247-afc812b9a356",
"title": "EP250427a / GRB 250427A: OHP/T193 photometric observations: third visit",
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"authors": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40306"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: OHP/T193 photometric observations: third visit",
"createdOn": 1746115933889,
"submitter": "Christophe Adami at LAM <christophe.adami@lam.fr>",
"circularId": 40306,
"submittedHow": "web"
},
"message_text": "C. Adami (LAM/Pytheas/AMU), S. Basa (Pytheas/OHP/LAM), N.A. Rakotondrainibe (LAM), S. Vergani (CNRS, Obs. de Paris, LUX), A. Saccardi (CEA/Irfu), B. Schneider (LAM), J. T. Palmerio (CEA/Irfu) report on behalf of the MISTRAL-GRB collaboration:\n\nWe observed EP250427a / GRB 250427A (Wang et al. GCN 40257, Ravasio et al. GCN 40262) a third time using the T193cm telescope at Observatoire de Haute-Provence (France) equipped with the MISTRAL spectro-imager in blue setting. \n\nWe obtained 1x72sec + 1x600sec + 1x900sec + 4x720sec in the r’ band starting at 01:34:13UT on 2025-05-01 (T0+93.9 h after the trigger). In the stacked image, we do not significantly detect the optical afterglow reported by Pérez-García et al. GCN 40259; Liu et al. GCN 40260; Becerra et al. GCN 40261; Brivio et al. GCN 40264; Chornock et al. GCN 40265; Saccardi et al. GCN 40266; Pérez-Fournon et al. GCN 40267; Xi et al. GCN 40269; Swain et al. GCN 40270; Magnani et al. GCN 40271; de Wet et al. GCN 40272; Busmann et al. GCN 40273, Wang et al. GCN40274, Amram et al. GCN 40275, Siegel et al. GCN 40279, Amram et al. GCN 40288, Eappachen et al. GCN 40289, and Ghosh et al. GCN 40293 down to the following 5-sigma upper limit: r > 22.8.\nIf we force the detection at the EP250427a afterglow position, we get a very marginal detection just below the detection limit with a preliminary magnitude of r=23.1+/-0.35\n\nThe photometric calibration was performed using nearby stars from the PanSTARRS catalog and the magnitude is not corrected for Galactic extinction.\n\nWe acknowledge the excellent support from the Observatoire de Haute-Provence and in particular Jean Balcaen, and the SOPHIE observer Owin Scutt.\n",
"published": "2025-05-10T16:09:53.497749Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-10T16:09:53.497767Z",
"modified": "2025-05-10T16:09:53.500990Z",
"nonlocalizedevents": [],
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},
{
"id": 196701,
"topic": "gcn.circulars",
"uuid": "44f5640d-de9d-47e9-bc02-3e1887163ee6",
"title": "EP250427a/GRB250427A: VLA Detection of Radio Counterpart",
"submitter": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"authors": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40488"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"eventId": "EP250427a",
"subject": "EP250427a/GRB250427A: VLA Detection of Radio Counterpart",
"createdOn": 1747624459936,
"submitter": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"circularId": 40488,
"submittedHow": "web"
},
"message_text": "Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report on behalf of a large collaboration. \n\nWe report the detection of a radio counterpart to the fast X-ray transient EP250427a, discovered by the Einstein Probe on 2025 April 27 (Wang et al., GCN 40257). EP250427a, also designated GRB250427A (Ravasio et al. GCN 40262), is a transient X-ray source notable for its rapid rise and decay in X-ray emission, peaking within minutes of detection. This behavior, combined with its potential association with a gamma-ray burst (GRB), suggests it may belong to a rare class of high-energy astrophysical phenomena. Initial follow-up observations in X-rays and optical wavelengths reported a faint, uncatalogued host galaxy at its position (Liu et al., GCN 40260), with subsequent spectroscopic observations confirming the host galaxy redshift of z=1.52 (Saccardi et al., GCN 40266).\n\nOn 2025 May 5, at approximately UT 11:25 (mid-time of the observation), we conducted observations using the Karl G. Jansky Very Large Array (VLA) under DDT program VLA/25A-467. The observations were carried out at central frequencies of 6 GHz (C-band) and 10 GHz (X-band). We detected a radio source coincident with the position of EP250427a, with measured flux densities of 26 +/- 7 μJy at 6 GHz and 77 +/- 7 μJy at 10 GHz. These values yield an inverted radio spectrum with a spectral index \\alpha ≈ 2.1 (where S \\propto ν^\\alpha), hinting at physical processes such as synchrotron self-absorption.\n\nThe inverted spectrum is consistent with an early-time radio afterglow where the emission region is still compact. Continued VLA monitoring is planned to track the flux density and spectral evolution of the radio counterpart, which will help refine models of its emission mechanism and constrain the energetics of the event.\n\nWe express our gratitude to the VLA TAC for approving the DDT proposal and to the VLA staff for their swift scheduling and support during these observations.",
"published": "2025-05-19T03:14:29.619523Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-19T03:14:29.619555Z",
"modified": "2025-05-19T03:14:29.631142Z",
"nonlocalizedevents": [],
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},
{
"id": 5787572,
"topic": "gcn.circulars",
"uuid": "1bf655d8-aad0-48e1-b7c5-67832d856e65",
"title": "EP250427a/GRB250427A: VLA Detection of Radio Counterpart",
"submitter": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"authors": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40488"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.40488....1A",
"eventId": "EP250427a",
"subject": "EP250427a/GRB250427A: VLA Detection of Radio Counterpart",
"createdOn": 1747624459936,
"submitter": "Tao An at SHAO, CAS <antao@shao.ac.cn>",
"circularId": 40488,
"submittedHow": "web"
},
"message_text": "Tao An and Yuanqi Liu (Shanghai Astronomical Observatory) report on behalf of a large collaboration. \n\nWe report the detection of a radio counterpart to the fast X-ray transient EP250427a, discovered by the Einstein Probe on 2025 April 27 (Wang et al., GCN 40257). EP250427a, also designated GRB250427A (Ravasio et al. GCN 40262), is a transient X-ray source notable for its rapid rise and decay in X-ray emission, peaking within minutes of detection. This behavior, combined with its potential association with a gamma-ray burst (GRB), suggests it may belong to a rare class of high-energy astrophysical phenomena. Initial follow-up observations in X-rays and optical wavelengths reported a faint, uncatalogued host galaxy at its position (Liu et al., GCN 40260), with subsequent spectroscopic observations confirming the host galaxy redshift of z=1.52 (Saccardi et al., GCN 40266).\n\nOn 2025 May 5, at approximately UT 11:25 (mid-time of the observation), we conducted observations using the Karl G. Jansky Very Large Array (VLA) under DDT program VLA/25A-467. The observations were carried out at central frequencies of 6 GHz (C-band) and 10 GHz (X-band). We detected a radio source coincident with the position of EP250427a, with measured flux densities of 26 +/- 7 μJy at 6 GHz and 77 +/- 7 μJy at 10 GHz. These values yield an inverted radio spectrum with a spectral index \\alpha ≈ 2.1 (where S \\propto ν^\\alpha), hinting at physical processes such as synchrotron self-absorption.\n\nThe inverted spectrum is consistent with an early-time radio afterglow where the emission region is still compact. Continued VLA monitoring is planned to track the flux density and spectral evolution of the radio counterpart, which will help refine models of its emission mechanism and constrain the energetics of the event.\n\nWe express our gratitude to the VLA TAC for approving the DDT proposal and to the VLA staff for their swift scheduling and support during these observations.",
"published": "2025-10-13T08:08:59.570392Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-10-13T08:08:59.570397Z",
"modified": "2025-10-13T08:08:59.575641Z",
"nonlocalizedevents": [],
"targets": []
},
{
"id": 194859,
"topic": "gcn.circulars",
"uuid": "9c8d27c0-3d48-404f-8b5f-14857b4f5196",
"title": "EP250427a / GRB 250427A: VLT/X-shooter redshift confirmation z = 1.520",
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"authors": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/40266"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.3.2/gcn/circulars.schema.json",
"bibcode": "2025GCN.40266....1S",
"eventId": "EP250427a",
"subject": "EP250427a / GRB 250427A: VLT/X-shooter redshift confirmation z = 1.520",
"createdOn": 1745768387938,
"submitter": "Daniele B. Malesani at IMAPP / Radboud University <d.malesani@astro.ru.nl>",
"circularId": 40266,
"submittedHow": "web"
},
"message_text": "A. Saccardi (CEA/Irfu), V. Abril-Melgarejo (LUX-Paris Obs.), Z. P. Zhu (NAOC), V. D’Elia (ASI/SSDC), A. J. Levan (Radboud and Warwick), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), B. Schneider (LAM), N. R. Tanvir (Leicester), A. L. Thakur (INAF/IAPS), S. D. Vergani (LUX-Paris Obs.), K. Wiersema (Hertfordshire), D. Xu (NAOC), report on behalf of the Stargate collaboration:\n\nWe observed the optical counterpart (Perez-Garcia et al., GCN 40259; Liu et al., GCN 40260; Becerra et al., GCN 40261; Brivio et al., GCN 40264) of EP250427a / GRB 250427A (Wang et al., GCN 40257; Ravasio et al., GCN 40262) using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph. Our spectra cover the wavelength range 3000-21,000 AA, and consist of 2 exposures of 600 s each. The observation mid time was 2025 Apr 27.394 UT (5.80 hr after the GRB).\n\nIn a 30-s image taken in the r band at a mid time of 5.61 hr after the trigger, we measure a magnitude r = 18.40 +- 0.02 AB, calibrated against nearby stars from the Pan-STARRS catalog.\n\nIn a preliminary reduction of the spectra, we clearly observe a bright continuum over the entire covered wavelength range, detected at high S/N. The sightline is rich with intervening absorption systems, in particular C IV absorbers (1548,1550 doublet) are detected at z = 1.146, 1.222, 1.231, 1.405, 1.407, 1.518, and 1.520.\n\nThe highest-redshift system is only securely detected in C IV and Si IV (at both z = 1.518 and 1.520). There is a possible indication of Lyman alpha absorption at the very blue end of the spectrum, with very low column density (much less than a DLA). The system at z = 1.405/1.407 is on the other hand the strongest and has detection in many species at both high and low ionization, including Si II, C II, Si IV, C IV, Fe II, Al II, Al III, Mg II and Mg I, as well as a rich velocity structure. No emission lines are visible at any of the above mentioned redshifts.\n\nOur data are therefore in good agreement with the results and the redshift value z = 1.520 already reported by Chornock et al. (GCN 40265) using the Keck telescope.\n\nWe acknowledge expert and efficient support from the observing staff at Paranal, in particular Boris Haeussler, Francesca Lucertini, Rodrigo Romero, and Elisa Garro.",
"published": "2025-05-12T08:09:28.838050Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-05-12T08:09:28.838066Z",
"modified": "2025-05-12T08:09:28.841764Z",
"nonlocalizedevents": [],
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}
]
}