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"time_created": "2025-08-30T10:43:41Z",
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},
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"published": "2025-08-30T10:43:42.585353Z",
"message_parser": "IGWN Alert Avro Parser v1",
"retracted": false,
"created": "2025-08-30T10:43:42.585369Z",
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}
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{
"id": 10091,
"sequence_number": 4,
"sequence_type": "UPDATE",
"message": {
"id": 5773938,
"topic": "igwn.gwalert",
"uuid": "98df30ed-b5df-47b8-bb21-c25147b25bda",
"title": "S250830bp - UPDATE",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
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"time_created": "2025-08-30T12:51:39Z",
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"published": "2025-08-30T12:51:40.057195Z",
"message_parser": "IGWN Alert Avro Parser v1",
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"modified": "2025-08-30T12:51:40.061368Z"
}
}
],
"references": [
{
"id": 5785262,
"topic": "gcn.circulars",
"uuid": "afeccdfb-f53a-46ac-ac9b-3bc273db976b",
"title": " LIGO/Virgo/KAGRA S250830bp: SOAR spectroscopy of two DESGW candidates",
"submitter": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"authors": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/42033"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.42033....1B",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": " LIGO/Virgo/KAGRA S250830bp: SOAR spectroscopy of two DESGW candidates",
"createdOn": 1759266690074,
"submitter": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"circularId": 42033,
"submittedHow": "web"
},
"message_text": "Hemanth Bommireddy (U de Chile), Regis Cartier (UA), Felipe Olivares (U Hawaii), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) spectroscopy team:\n\n\nWe report on AT2025wzx and AT2025wpo, which were observed by SOAR 2025-09-10T03:10:05 UTC (PI Bommireddy). We clearly see broad Halpha and Hbeta in AT2025wzx, securing its classification as an AGN at z~0.23, thus, we therefore conclude this candidate to be far too distant to be associated with S250830bp (GCN 41607). In contrast, AT2025wpo shows a noisy featureless spectrum, preventing a firm classification.\n\nWe thank SOAR Staff for supporting these observations.\n",
"published": "2025-10-06T08:09:14.069799Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-10-06T08:09:14.069821Z",
"modified": "2025-10-06T08:09:14.074629Z",
"targets": []
},
{
"id": 5783432,
"topic": "gcn.circulars",
"uuid": "6bebc57d-a1e2-413c-b652-d653a01d6134",
"title": " LIGO/Virgo/KAGRA S250830bp: SOAR spectroscopy of two DESGW candidates",
"submitter": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"authors": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/42033"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": " LIGO/Virgo/KAGRA S250830bp: SOAR spectroscopy of two DESGW candidates",
"createdOn": 1759266690074,
"submitter": "Hemanth kumar <hemanth.bommireddy195@gmail.com>",
"circularId": 42033,
"submittedHow": "web"
},
"message_text": "Hemanth Bommireddy (U de Chile), Regis Cartier (UA), Felipe Olivares (U Hawaii), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) spectroscopy team:\n\n\nWe report on AT2025wzx and AT2025wpo, which were observed by SOAR 2025-09-10T03:10:05 UTC (PI Bommireddy). We clearly see broad Halpha and Hbeta in AT2025wzx, securing its classification as an AGN at z~0.23, thus, we therefore conclude this candidate to be far too distant to be associated with S250830bp (GCN 41607). In contrast, AT2025wpo shows a noisy featureless spectrum, preventing a firm classification.\n\nWe thank SOAR Staff for supporting these observations.\n",
"published": "2025-09-30T21:11:39.202670Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-30T21:11:39.202685Z",
"modified": "2025-09-30T21:11:39.208929Z",
"targets": []
},
{
"id": 5781057,
"topic": "gcn.circulars",
"uuid": "739c2548-4a10-4270-851f-7dc4a8a52a80",
"title": "LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume",
"submitter": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"authors": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41608"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41608....1C",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume",
"createdOn": 1756561797236,
"submitter": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"circularId": 41608,
"submittedHow": "web"
},
"message_text": "David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)\n\nOn behalf of the NASA/IPAC Extragalactic Database (NED) Team.\n\nWe spatially cross-matched the LVK S250830bp-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 72 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:\n\nFull List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4 \nTop 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4/20\n\nThe NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.\n\n| objname| ra| dec|objtype| DistMpc|DistMpc_unc| m_NUV| m_NUV_unc| m_Ks| m_Ks_unc| m_W1| m_W1_unc| P_3D|P_3D_LumW1|\n|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|\n|WISEA J215127.50-771123.7| 327.86462| -77.18992| G| 452.47| 0.65| null| null| 12.307| 0.126| 10.624| 0.006|1.78e-05| 2.49e-06|\n|WISEA J214848.24-774454.9| 327.20101| -77.74860| G| 463.54| null| null| null| 13.779| 0.215| 12.525| 0.010|5.02e-05| 1.28e-06|\n|WISEA J214657.45-773648.1| 326.73941| -77.61338| G| 462.69| null| null| null| 13.264| 0.192| 12.822| 0.014|5.09e-05| 9.83e-07|\n|WISEA J215919.74-775235.8| 329.83229| -77.87662| G| 451.54| 0.65| null| null| 12.506| 0.114| 12.064| 0.008|1.84e-05| 6.79e-07|\n|WISEA J213625.95-773427.9| 324.10814| -77.57443| G| 382.59| null| null| null| 13.364| 0.175| 12.873| 0.009|5.20e-05| 6.48e-07|\n|WISEA J214445.54-771938.0| 326.18978| -77.32723| G| 452.13| null| 21.430| 0.312| 13.324| 0.160| 13.240| 0.012|4.93e-05| 6.16e-07|\n|WISEA J213608.67-772811.0| 324.03613| -77.46974| G| 356.09| null| null| null| 13.524| 0.185| 12.576| 0.014|4.08e-05| 5.79e-07|\n|WISEA J215658.48-774037.0| 329.24368| -77.67696| G| 403.25| null| null| null| 13.090| 0.145| 12.789| 0.009|3.68e-05| 5.55e-07|\n|WISEA J213700.75-773426.0| 324.25313| -77.57391| G| 325.21| 0.65| 20.310| 0.217| 12.016| 0.075| 11.838| 0.006|2.20e-05| 5.14e-07|\n|WISEA J213454.05-772745.8| 323.72522| -77.46275| G| 372.81| null| null| null| 13.059| 0.143| 13.016| 0.016|4.75e-05| 4.92e-07|\n|WISEA J215146.52-780557.3| 327.94386| -78.09927| G| 368.25| 0.65| null| null| 12.882| 0.135| 12.228| 0.010|2.20e-05| 4.63e-07|\n|WISEA J213733.97-780150.6| 324.39156| -78.03074| G| 345.10| null| 20.443| 0.108| 13.417| 0.204| 11.831| 0.007|1.74e-05| 4.63e-07|\n|WISEA J212943.06-775542.2| 322.42945| -77.92839| G| 372.02| 0.65| null| null| 12.273| 0.114| 12.187| 0.009|1.85e-05| 4.12e-07|\n|WISEA J213608.13-772733.1| 324.03389| -77.45922| G| 319.73| 0.65| 21.351| 0.351| 12.228| 0.076| 11.985| 0.007|2.01e-05| 3.96e-07|\n|WISEA J212913.11-773431.7| 322.30466| -77.57550| G| 378.32| 0.65| 20.656| 0.234| 13.354| 0.165| 12.886| 0.013|2.92e-05| 3.50e-07|\n|WISEA J213641.68-775334.3| 324.17371| -77.89287| G| 411.08| null| 21.445| 0.179| 13.521| 0.179| 13.546| 0.029|4.09e-05| 3.18e-07|\n|WISEA J213632.95-765253.9| 324.13731| -76.88165| G| 460.55| 0.65| null| null| 12.341| 0.134| 12.450| 0.009|1.07e-05| 2.86e-07|\n|WISEA J213724.56-773623.7| 324.35235| -77.60660| G| 536.58| null| null| null| 13.729| 0.212| 13.039| 0.022|1.35e-05| 2.84e-07|\n|WISEA J215903.81-773020.5| 329.76589| -77.50571| G| 426.05| null| null| null| 13.020| 0.144| 12.826| 0.010|1.74e-05| 2.83e-07|\n|WISEA J212501.23-771251.5| 321.25514| -77.21431| G| 436.36| null| null| null| 13.307| 0.143| 12.903| 0.015|1.69e-05| 2.66e-07|\n\nTable 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250830bp sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). \n",
"published": "2025-09-22T08:08:52.609203Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:52.609212Z",
"modified": "2025-09-22T08:08:52.612750Z",
"targets": []
},
{
"id": 5781036,
"topic": "gcn.circulars",
"uuid": "4529ea91-0d28-4be3-afbf-1ee3d907cbde",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam/SOAR GW-MMADS updates",
"submitter": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"authors": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41741"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41741....1H",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam/SOAR GW-MMADS updates",
"createdOn": 1757178095293,
"submitter": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"circularId": 41741,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), James Freeburn (UNC), Antonella Palmese (CMU), Brendan O’Connor (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:\n\nDECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-05T04:12:03 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS 13 newly identified transients within the LVK 99% CI area. We report the new transient AT 2025xab as a likely supernova and note that none of our nuclear transients show any compelling evolution.\n\nA crossmatch with Gaia reveals AT 2025wzv and AT 2025wzw (McMahon et al. GCN 41739) as galactic sources, thus we conclude these are not associated with S250830bp (GCN 41607).\n\nWe also provide updates on previously announced candidates:\n\nWe report that AT 2025wpk has shown little to no color evolution or brightness change since its last observation and that AT 2025wpv has shown a slow decline in brightness, dropping ~0.1 mag/day.\n\nFinally, we report on AT 2025wpq (GCN 41643), which was observed by SOAR at 2025-09-04T05:32:07 UTC (PI Andreoni). We clearly see broad Halpha and Hbeta securing its classification as a quasar at a z ~ 0.4, thus, we therefore conclude this candidate is far too distant to be associated with S250830bp (GCN 41607).\n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-09-22T08:08:50.387642Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:50.387650Z",
"modified": "2025-09-22T08:08:50.391078Z",
"targets": []
},
{
"id": 5781022,
"topic": "gcn.circulars",
"uuid": "9834532d-db36-4c2d-a5ba-a02a87a112ed",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41640"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41640....1M",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates",
"createdOn": 1756758772952,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41640,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-09-01 03:39:18 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates:\n| RA | DEC |\n|----------|----------|\n| 324.165777 | -77.51123 |\n| 327.955221 | -77.54517 |\n| 323.965777 | -77.61123 |\n| 327.755221 | -77.64517 |\n\nThese pointings cover the 90% localization region of candidate gravitational-wave event S250830bp.\n\nAll fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 in z.\n\nWe process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.\n\nPoor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift.\n\n| ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- |\n| DESGW_3336579 | AT2025wpk | 329.476163 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 |\n\nFurther observations are ongoing and we encourage followup of the event region and the above candidate.\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-09-22T08:08:50.248842Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:50.248850Z",
"modified": "2025-09-22T08:08:50.252280Z",
"targets": []
},
{
"id": 5780899,
"topic": "gcn.circulars",
"uuid": "026e2901-3d3f-42bd-a1e4-29b28d38e80f",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 2 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41739"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41739....1M",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 2 Candidates",
"createdOn": 1757165834822,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41739,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-09-06 04:12:03 UTC, the Dark Energy Camera (DECam) conducted the second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed the same fields as epoch 1 (GCN 41640) and employed the same analysis. The average limiting magnitudes achieved for epoch 2 were 20.7 in g, 21.4 in r, and 21.3 in i, and 21.1 in z.\n\nWe report the following seven candidates which have multiple good detections on both days of observation. The candidate reported previously, AT2025wpk, is confirmed to be variable and dimming with good detections in all four bands. The other six candidates are all nuclear-type transients at the centers of previously detected objects.\n\n| ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- |\n| DESGW_3336579 | AT2025wpk | 329.476163 | -77.412560 | 22.43 | 0.24 | 21.58 | 0.19 | 21.30 | 0.13 | 21.05 | 0.09 |\n| DESGW_3336526 | AT2025wzu | 325.368195 | -77.603585 | N/A | N/A | 21.16 | 0.09 | N/A | N/A | 21.76 | 0.17 |\n| DESGW_3336621 | AT2025wzw | 323.031060 | -77.708095 | 23.21 | 0.49 | 21.74 | 0.23 | 21.52 | 0.10 | 20.68 | 0.07 |\n| DESGW_3336660 | AT2025wzv | 328.307211 | -78.281736 | 15.94 | 0.01 | N/A | N/A | N/A | N/A | N/A | N/A |\n| DESGW_3336681 | AT2025wpr | 321.643320 | -77.755092 | N/A | N/A | 21.07 | 0.08 | 21.72 | 0.14 | 20.98 | 0.10 |\n| DESGW_3336921 | AT2025wpo | 323.141341 | -78.204743 | 20.22 | 0.06 | 20.27 | 0.06 | 20.32 | 0.06 | 20.38 | 0.06 |\n| DESGW_3337296 | AT2025wzx | 321.552602 | -78.046756 | N/A | N/A | 21.77 | 0.16 | 20.95 | 0.07 | 21.65 | 0.19 |\n\nIn particular, AT2025wzv seems to be very luminous and actively brightening in g-band but shows little to no transient activity in other bands. This behavior makes it an interesting transient but unlikely to be associated with S250830bp.\n\nWe also recover the transient AT2025wpq reported in Hall et al (GCN 41643), although we note that the redshift reported for the host AGN by the Quaia catalog (Storey-Fisher et al 2024) is z = 0.43 +- 0.06, outside of the confidence volume for S250830bp. We do not recover the transient AT2025wpv.\n\nFurther observations are ongoing and we encourage followup of the event region and the above candidates.\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-09-22T08:08:48.010287Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:48.010295Z",
"modified": "2025-09-22T08:08:48.014170Z",
"targets": []
},
{
"id": 5780885,
"topic": "gcn.circulars",
"uuid": "99b6ec0f-4d57-4c79-a213-f1230a1fd055",
"title": "LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations",
"submitter": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"authors": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41610"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41610....1M",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations",
"createdOn": 1756562737515,
"submitter": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"circularId": 41610,
"submittedHow": "web"
},
"message_text": "T. Mihara, N. Kawai (RIKEN),\nM. Nakajima, H. Negoro, K. Takagi (Nihon U.),\nS. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)\nreport on behalf of the MAXI team:\n\nWe examined MAXI/GSC all-sky X-ray images (2-20 keV)\nafter compact binary merger candidate S250830bp at 2025-08-30 10:24:18.852 UTC (GCN 41606, 41607).\n\nAt the trigger time of S250830bp, the high-voltage of MAXI/GSC was on,\nbut the FOV was out of the 90% credible region of the Bilby skymap.\nThe credible region was observed about 3 minutes before the GW trigger time with GSC, and \nthe first one-orbit (92 min) scan observation after the event covered 100%\nof the 90% credible region from 11:52:34 to 11:53:19 UTC (T0+5296 to T0+5341 sec).\n\nNo significant new source was found in the region in those observations.\nA typical 1-sigma averaged upper limit obtained in one scan observation\nis 20 mCrab at 2-20 keV.\n\nIf you require information about X-ray flux by MAXI/GSC at specific coordinates,\nplease contact the submitter of this circular by email.\n",
"published": "2025-09-22T08:08:46.867349Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:46.867358Z",
"modified": "2025-09-22T08:08:46.870654Z",
"targets": []
},
{
"id": 5780884,
"topic": "gcn.circulars",
"uuid": "93a89282-27ae-4fc2-99d6-7e1aee35aa8e",
"title": "LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume",
"submitter": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"authors": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41609"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41609....1B",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume",
"createdOn": 1756562512577,
"submitter": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"circularId": 41609,
"submittedHow": "web"
},
"message_text": "M. L. Brozzetti (UniPG/INFN), G. Dálya (L2IT/EotvosU), G. Greco (INFN), M. Bawaj (UniPG/INFN), T. Matcovich (UniPG/INFN), S. Cutini (INFN) , R. De Pietri (UniPR/INFN), Marica Branchesi (GSSI)\n\nOn behalf of the GLADEnet Team.\n\nWe analyzed the completeness of the GLADE+ [1] catalog within the 90% credible localization volume of the S250830bp event from the 4-Update alert from the GCN Circular 41606.\n\nThe completeness value is 6.6e-1 in the B-band using the last released skymap : [Bilby.multiorder.fits,0](https://gracedb.ligo.org/api/superevents/S250830bp/files/Bilby.multiorder.fits,0), which means that the catalogue contains 66% of the total light in the B-band expected from galaxies in the localization volume. \n\nA total of 517 galaxies are identified within the 90% gravitational volume. The complete list of galaxies can be downloaded from the GLADEnet webpage [2] : [https://virgo.pg.infn.it/gladenet/catalogs/](https://virgo.pg.infn.it/gladenet/catalogs/)\n\nGLADEnet allows for the interactive visualization of the 90% localization area and its intersection with regions of high extinction as defined in GLADE+. Furthermore, the first 1000 galaxies can be explored interactively, enabling users to filter galaxies based on their 3D probability density or their absolute B magnitude. The ligo.skymap cross-match method [3,4] is used to obtain the list of galaxies.\n\nReferences:\n\n[1]GLADE+: An Extended Galaxy Catalogue for Multimessenger Searches with Advanced Gravitational-wave Detectors G. Dálya et al. MNRAS, 514,1, pp.1403-1411, 2022 \n\n[2] GLADEnet: A progressive web app for multi-messenger cosmology and electromagnetic follow-ups of gravitational-wave sources M. L. Brozzetti, G. Dálya, G. Greco, M. Bawaj, T. Matcovich, M. Branchesi, T. Boch, M. Baumann, S. Cutini, R. De Pietri et al. (4 more) A&A, 684, A44 (2024)\n\n[3] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Lett., 829, L15. doi:10.3847/2041-8205/829/1/L15\n\n[4] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Supp., 226, 10. doi:10.3847/0067-0049/226/1/10\n",
"published": "2025-09-22T08:08:46.850650Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:46.850658Z",
"modified": "2025-09-22T08:08:46.854078Z",
"targets": []
},
{
"id": 5780883,
"topic": "gcn.circulars",
"uuid": "395214d9-0cc7-44cf-bf41-5cdfe444532a",
"title": "LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate",
"submitter": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"authors": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41606"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41606....1L",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate",
"createdOn": 1756551306846,
"submitter": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"circularId": 41606,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe identified the compact binary merger candidate S250830bp during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-30 10:24:18.852 UTC (GPS time: 1440584676.852). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], MLy [6], PyCBC Live [7], and SPIIR [8] analysis pipelines.\n\nS250830bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:\n\nhttps://gracedb.ligo.org/superevents/S250830bp\n\nThe classification of the GW signal, in order of descending probability, is BBH (84%), NSBH (16%), Terrestrial (<1%), or BNS (<1%).\n\nAssuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 16%.\n\nThe source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.\n\nTwo sky maps are available at this time and can be retrieved from the GraceDB event page:\n * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [10], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.\n * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [11], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.\n\nThe preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 11 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 429 +/- 100 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010\n [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004\n [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018\n [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008\n [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234\n [6] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034\n [7] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a\n [8] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023\n [9] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe\n [10] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982\n [11] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013\n",
"published": "2025-09-22T08:08:46.834782Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:46.834790Z",
"modified": "2025-09-22T08:08:46.838401Z",
"targets": []
},
{
"id": 5780852,
"topic": "gcn.circulars",
"uuid": "826a11cb-ea51-4165-b355-9f63918680e0",
"title": "LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification",
"submitter": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"authors": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41607"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41607....1L",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification",
"createdOn": 1756558543716,
"submitter": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"circularId": 41607,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250830bp (GCN Circular 41606). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:\n\nhttps://gracedb.ligo.org/superevents/S250830bp\n\nAssuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability,\nis BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).\n\nBased on posterior support from parameter estimation [1], under the assumption that the candidate S250830bp is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.\n\nFor the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 4 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):\n icrs; ellipse(21h43m, -77d36m, 1.36d, 0.88d, 15.49d)\nMarginalized over the whole sky, the a posteriori luminosity distance estimate is 427 +/- 69 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040\n [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe",
"published": "2025-09-22T08:08:46.516853Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:46.516862Z",
"modified": "2025-09-22T08:08:46.520268Z",
"targets": []
},
{
"id": 5780807,
"topic": "gcn.circulars",
"uuid": "63308006-7e49-4920-a951-906c1d74993f",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates",
"submitter": "xjh@andrew.cmu.edu",
"authors": "xjh@andrew.cmu.edu",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41643"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41643....1H",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates",
"createdOn": 1756764292888,
"submitter": "xjh@andrew.cmu.edu",
"circularId": 41643,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:\n\nDECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest:\n\nFirst, one matched to a NED galaxy within the 90% volume (GCN 41608):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |\n| T202509012137497m775946 | AT 2025wpv | 324.457179 | -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6”\n\nNext, a transient of a known Gaia quasar (Gavras et al. 2023):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |\n| A202509012159035m780432 | AT 2025wpq | 329.764660 | -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165”\n\nFinally, we confirm the transient reported by McMahon et al. (GCN 41640):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |\n| T202509012157542m772445 | AT 2025wpk | 329.475736 | -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 \n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-09-22T08:08:41.970728Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-22T08:08:41.970737Z",
"modified": "2025-09-22T08:08:41.974116Z",
"targets": []
},
{
"id": 5776148,
"topic": "gcn.circulars",
"uuid": "bda3b3ae-644d-429d-b582-54709938a440",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam/SOAR GW-MMADS updates",
"submitter": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"authors": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41741"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam/SOAR GW-MMADS updates",
"createdOn": 1757178095293,
"submitter": "Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>",
"circularId": 41741,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), James Freeburn (UNC), Antonella Palmese (CMU), Brendan O’Connor (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:\n\nDECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-05T04:12:03 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS 13 newly identified transients within the LVK 99% CI area. We report the new transient AT 2025xab as a likely supernova and note that none of our nuclear transients show any compelling evolution.\n\nA crossmatch with Gaia reveals AT 2025wzv and AT 2025wzw (McMahon et al. GCN 41739) as galactic sources, thus we conclude these are not associated with S250830bp (GCN 41607).\n\nWe also provide updates on previously announced candidates:\n\nWe report that AT 2025wpk has shown little to no color evolution or brightness change since its last observation and that AT 2025wpv has shown a slow decline in brightness, dropping ~0.1 mag/day.\n\nFinally, we report on AT 2025wpq (GCN 41643), which was observed by SOAR at 2025-09-04T05:32:07 UTC (PI Andreoni). We clearly see broad Halpha and Hbeta securing its classification as a quasar at a z ~ 0.4, thus, we therefore conclude this candidate is far too distant to be associated with S250830bp (GCN 41607).\n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-09-06T17:01:42.276074Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-06T17:01:42.276096Z",
"modified": "2025-09-06T17:01:42.292716Z",
"targets": []
},
{
"id": 5776111,
"topic": "gcn.circulars",
"uuid": "2e0e1bf7-ba24-4549-be82-d594671e2ba2",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 2 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41739"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 2 Candidates",
"createdOn": 1757165834822,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41739,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-09-06 04:12:03 UTC, the Dark Energy Camera (DECam) conducted the second epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed the same fields as epoch 1 (GCN 41640) and employed the same analysis. The average limiting magnitudes achieved for epoch 2 were 20.7 in g, 21.4 in r, and 21.3 in i, and 21.1 in z.\n\nWe report the following seven candidates which have multiple good detections on both days of observation. The candidate reported previously, AT2025wpk, is confirmed to be variable and dimming with good detections in all four bands. The other six candidates are all nuclear-type transients at the centers of previously detected objects.\n\n| ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- |\n| DESGW_3336579 | AT2025wpk | 329.476163 | -77.412560 | 22.43 | 0.24 | 21.58 | 0.19 | 21.30 | 0.13 | 21.05 | 0.09 |\n| DESGW_3336526 | AT2025wzu | 325.368195 | -77.603585 | N/A | N/A | 21.16 | 0.09 | N/A | N/A | 21.76 | 0.17 |\n| DESGW_3336621 | AT2025wzw | 323.031060 | -77.708095 | 23.21 | 0.49 | 21.74 | 0.23 | 21.52 | 0.10 | 20.68 | 0.07 |\n| DESGW_3336660 | AT2025wzv | 328.307211 | -78.281736 | 15.94 | 0.01 | N/A | N/A | N/A | N/A | N/A | N/A |\n| DESGW_3336681 | AT2025wpr | 321.643320 | -77.755092 | N/A | N/A | 21.07 | 0.08 | 21.72 | 0.14 | 20.98 | 0.10 |\n| DESGW_3336921 | AT2025wpo | 323.141341 | -78.204743 | 20.22 | 0.06 | 20.27 | 0.06 | 20.32 | 0.06 | 20.38 | 0.06 |\n| DESGW_3337296 | AT2025wzx | 321.552602 | -78.046756 | N/A | N/A | 21.77 | 0.16 | 20.95 | 0.07 | 21.65 | 0.19 |\n\nIn particular, AT2025wzv seems to be very luminous and actively brightening in g-band but shows little to no transient activity in other bands. This behavior makes it an interesting transient but unlikely to be associated with S250830bp.\n\nWe also recover the transient AT2025wpq reported in Hall et al (GCN 41643), although we note that the redshift reported for the host AGN by the Quaia catalog (Storey-Fisher et al 2024) is z = 0.43 +- 0.06, outside of the confidence volume for S250830bp. We do not recover the transient AT2025wpv.\n\nFurther observations are ongoing and we encourage followup of the event region and the above candidates.\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-09-06T13:37:23.888216Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-06T13:37:23.888236Z",
"modified": "2025-09-06T13:37:23.897317Z",
"targets": []
},
{
"id": 5774572,
"topic": "gcn.circulars",
"uuid": "19807d97-1cdb-4224-9b56-880431e2c9ec",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates",
"submitter": "xjh@andrew.cmu.edu",
"authors": "xjh@andrew.cmu.edu",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41643"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam GW-MMADS candidates",
"createdOn": 1756764292888,
"submitter": "xjh@andrew.cmu.edu",
"circularId": 41643,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Brendan O’Connor (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), James Freeburn (UNC), Keerthi Kunnumkai (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team report:\n\nDECam observed the high probability area of the LVK gravitational wave candidate S250830bp (GCN 41606) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope. Observations started at 2025-09-01T03:39 UTC (Prop ID: 2025B-485252; PI Soares-Santos) and covered the highest 90% probability region of the event (GCN 41607).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and then visually inspect the remaining transients. We report on TNS new transients within the LVK 99% CI area, and we report here select transients of high quality and interest:\n\nFirst, one matched to a NED galaxy within the 90% volume (GCN 41608):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | NED separation |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |\n| T202509012137497m775946 | AT 2025wpv | 324.457179 | -77.996131 | 2025-09-01 03:57:43.776 | 22.6 | 0.3 | 1.16 | 1.6”\n\nNext, a transient of a known Gaia quasar (Gavras et al. 2023):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | Gaia separation |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- | ----- |\n| A202509012159035m780432 | AT 2025wpq | 329.764660 | -78.075678 | 2025-09-01 03:41:20.544 | 20.3 | 0.3 | 0.6 | 0.165”\n\nFinally, we confirm the transient reported by McMahon et al. (GCN 41640):\n\n| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |\n| T202509012157542m772445 | AT 2025wpk | 329.475736 | -77.412522 | 2025-09-01 03:49:33.024 | 22.2 | 0.2 | 1.2 \n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-09-01T22:05:01.611480Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-01T22:05:01.611497Z",
"modified": "2025-09-01T22:05:01.622181Z",
"targets": []
},
{
"id": 5774555,
"topic": "gcn.circulars",
"uuid": "369f2fec-043f-4d0e-97ef-20ae7f1ffcfd",
"title": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41640"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: DECam DESGW Epoch 1 Candidates",
"createdOn": 1756758772952,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41640,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Lillian Joseph (Benedictine U.), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-09-01 03:39:18 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250830bp (GCN 41606). We observed four fields centered on the following ICRS coordinates:\n| RA | DEC |\n|----------|----------|\n| 324.165777 | -77.51123 |\n| 327.955221 | -77.54517 |\n| 323.965777 | -77.61123 |\n| 327.755221 | -77.64517 |\n\nThese pointings cover the 90% localization region of candidate gravitational-wave event S250830bp.\n\nAll fields were observed in DECam g, r, i, z filters with 90-second exposures. The limiting magnitudes achieved for epoch 1 were 21.8 in g, 21.8 in r, and 21.5 in i, and 20.7 in z.\n\nWe process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, and Quaia AGN catalogs (Secrest et al 2015, Flesch 2023, Storey-Fisher et al 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.\n\nPoor observing conditions during observing complicated positive identification of transient candidates. However, we report one candidate below with good detections in multiple bands. The candidate is separated by 5.58 arcseconds from the center of the apparent host galaxy LEDA 236249, which does not have a reported redshift.\n\n| ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|--------- |-------- |------------ |----------- |------- |----------- |------- |----------- |------- |----------- |------- |----------- |\n| DESGW_3336579 | AT2025wpk | 329.476163 | -77.41256 | 22.41 | 0.24 | 21.39 | 0.09 | 21.1 | 0.06 | 20.93 | 0.14 |\n\nFurther observations are ongoing and we encourage followup of the event region and the above candidate.\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-09-01T20:33:02.074098Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-09-01T20:33:02.074115Z",
"modified": "2025-09-01T20:33:02.090648Z",
"targets": []
},
{
"id": 5773969,
"topic": "gcn.circulars",
"uuid": "4d972466-6f74-49ca-89b1-cafdef130104",
"title": "LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations",
"submitter": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"authors": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41610"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Coverage and upper limits from MAXI/GSC observations",
"createdOn": 1756562737515,
"submitter": "Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>",
"circularId": 41610,
"submittedHow": "web"
},
"message_text": "T. Mihara, N. Kawai (RIKEN),\nM. Nakajima, H. Negoro, K. Takagi (Nihon U.),\nS. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)\nreport on behalf of the MAXI team:\n\nWe examined MAXI/GSC all-sky X-ray images (2-20 keV)\nafter compact binary merger candidate S250830bp at 2025-08-30 10:24:18.852 UTC (GCN 41606, 41607).\n\nAt the trigger time of S250830bp, the high-voltage of MAXI/GSC was on,\nbut the FOV was out of the 90% credible region of the Bilby skymap.\nThe credible region was observed about 3 minutes before the GW trigger time with GSC, and \nthe first one-orbit (92 min) scan observation after the event covered 100%\nof the 90% credible region from 11:52:34 to 11:53:19 UTC (T0+5296 to T0+5341 sec).\n\nNo significant new source was found in the region in those observations.\nA typical 1-sigma averaged upper limit obtained in one scan observation\nis 20 mCrab at 2-20 keV.\n\nIf you require information about X-ray flux by MAXI/GSC at specific coordinates,\nplease contact the submitter of this circular by email.\n",
"published": "2025-08-30T14:05:39.013545Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-30T14:05:39.013561Z",
"modified": "2025-08-30T14:05:39.018074Z",
"targets": []
},
{
"id": 5773966,
"topic": "gcn.circulars",
"uuid": "b41436b8-da67-498f-aa00-6efd45f5e1f7",
"title": "LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume",
"submitter": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"authors": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41609"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp GLADEnet Completeness: Potential Host Galaxies in the 90% Credible Volume",
"createdOn": 1756562512577,
"submitter": "Maria Brozzetti at Università degli Studi di Perugia <marialisa.brozzetti@ligo.org>",
"circularId": 41609,
"submittedHow": "web"
},
"message_text": "M. L. Brozzetti (UniPG/INFN), G. Dálya (L2IT/EotvosU), G. Greco (INFN), M. Bawaj (UniPG/INFN), T. Matcovich (UniPG/INFN), S. Cutini (INFN) , R. De Pietri (UniPR/INFN), Marica Branchesi (GSSI)\n\nOn behalf of the GLADEnet Team.\n\nWe analyzed the completeness of the GLADE+ [1] catalog within the 90% credible localization volume of the S250830bp event from the 4-Update alert from the GCN Circular 41606.\n\nThe completeness value is 6.6e-1 in the B-band using the last released skymap : [Bilby.multiorder.fits,0](https://gracedb.ligo.org/api/superevents/S250830bp/files/Bilby.multiorder.fits,0), which means that the catalogue contains 66% of the total light in the B-band expected from galaxies in the localization volume. \n\nA total of 517 galaxies are identified within the 90% gravitational volume. The complete list of galaxies can be downloaded from the GLADEnet webpage [2] : [https://virgo.pg.infn.it/gladenet/catalogs/](https://virgo.pg.infn.it/gladenet/catalogs/)\n\nGLADEnet allows for the interactive visualization of the 90% localization area and its intersection with regions of high extinction as defined in GLADE+. Furthermore, the first 1000 galaxies can be explored interactively, enabling users to filter galaxies based on their 3D probability density or their absolute B magnitude. The ligo.skymap cross-match method [3,4] is used to obtain the list of galaxies.\n\nReferences:\n\n[1]GLADE+: An Extended Galaxy Catalogue for Multimessenger Searches with Advanced Gravitational-wave Detectors G. Dálya et al. MNRAS, 514,1, pp.1403-1411, 2022 \n\n[2] GLADEnet: A progressive web app for multi-messenger cosmology and electromagnetic follow-ups of gravitational-wave sources M. L. Brozzetti, G. Dálya, G. Greco, M. Bawaj, T. Matcovich, M. Branchesi, T. Boch, M. Baumann, S. Cutini, R. De Pietri et al. (4 more) A&A, 684, A44 (2024)\n\n[3] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Lett., 829, L15. doi:10.3847/2041-8205/829/1/L15\n\n[4] Singer, L. P., Chen, H.-Y., Holz, D. E., et al. 2016, Astropys. J. Supp., 226, 10. doi:10.3847/0067-0049/226/1/10\n",
"published": "2025-08-30T14:02:01.303546Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-30T14:02:01.303563Z",
"modified": "2025-08-30T14:02:01.309983Z",
"targets": []
},
{
"id": 5773965,
"topic": "gcn.circulars",
"uuid": "ffa46682-e721-4154-be88-af814980bd74",
"title": "LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume",
"submitter": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"authors": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41608"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: NED Galaxies in the 4-Update Localization Volume",
"createdOn": 1756561797236,
"submitter": "David Cook at Caltech/IPAC-NED <dcook@ipac.caltech.edu>",
"circularId": 41608,
"submittedHow": "web"
},
"message_text": "David O. Cook (Caltech/IPAC), Rick Ebert (Caltech/IPAC), George Helou (Caltech/IPAC), Joseph M. Mazzarella (Caltech/IPAC), Marion Schmitz (Caltech/IPAC), and Leo Singer (NASA/GSFC)\n\nOn behalf of the NASA/IPAC Extragalactic Database (NED) Team.\n\nWe spatially cross-matched the LVK S250830bp-4-Update sky localization with the NED Local Volume Sample (NED-LVS; Cook et al. 2023), which is a subset of NED with a redshift or redshift-independent distance less than 1000 Mpc. We find 72 galaxies within the 90% containment volume, and we list here the top 20 galaxies sorted by the joint probability of the 3D localization and the WISE W1 luminosity (an observable proxy for stellar mass). For the full or top 20 list of galaxies in the 90% volume go either to the NED Gravitational Wave Followup service at https://ned.ipac.caltech.edu/uri/NED::GWF/ or click on the following links:\n\nFull List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4 \nTop 20 List Download: https://ned.ipac.caltech.edu/uri/NED::GWFglist/fits/S250830bp/4/20\n\nThe NED-GWF service provides downloadable galaxy lists and visualizations for candidate host galaxies. For each GW alert, these products are automatically generated and made available within minutes to expedite efficient electromagnetic follow-up observations. The NED top 20 list is sorted by the joint probability of the 3D localization and the WISE W1 luminosity, but users can sort on additional pre-computed prioritization metrics (star formation rate, P_3D * P_SFR; and specific star formation rate, P_3D * P_sSFR; etc.) which are available via downloading the entire galaxy list inside the event's probability volume.\n\n| objname| ra| dec|objtype| DistMpc|DistMpc_unc| m_NUV| m_NUV_unc| m_Ks| m_Ks_unc| m_W1| m_W1_unc| P_3D|P_3D_LumW1|\n|-------------------------|--------------|--------------|-------|-----------|-----------|------------|------------|------------|------------|------------|------------|--------|----------|\n|WISEA J215127.50-771123.7| 327.86462| -77.18992| G| 452.47| 0.65| null| null| 12.307| 0.126| 10.624| 0.006|1.78e-05| 2.49e-06|\n|WISEA J214848.24-774454.9| 327.20101| -77.74860| G| 463.54| null| null| null| 13.779| 0.215| 12.525| 0.010|5.02e-05| 1.28e-06|\n|WISEA J214657.45-773648.1| 326.73941| -77.61338| G| 462.69| null| null| null| 13.264| 0.192| 12.822| 0.014|5.09e-05| 9.83e-07|\n|WISEA J215919.74-775235.8| 329.83229| -77.87662| G| 451.54| 0.65| null| null| 12.506| 0.114| 12.064| 0.008|1.84e-05| 6.79e-07|\n|WISEA J213625.95-773427.9| 324.10814| -77.57443| G| 382.59| null| null| null| 13.364| 0.175| 12.873| 0.009|5.20e-05| 6.48e-07|\n|WISEA J214445.54-771938.0| 326.18978| -77.32723| G| 452.13| null| 21.430| 0.312| 13.324| 0.160| 13.240| 0.012|4.93e-05| 6.16e-07|\n|WISEA J213608.67-772811.0| 324.03613| -77.46974| G| 356.09| null| null| null| 13.524| 0.185| 12.576| 0.014|4.08e-05| 5.79e-07|\n|WISEA J215658.48-774037.0| 329.24368| -77.67696| G| 403.25| null| null| null| 13.090| 0.145| 12.789| 0.009|3.68e-05| 5.55e-07|\n|WISEA J213700.75-773426.0| 324.25313| -77.57391| G| 325.21| 0.65| 20.310| 0.217| 12.016| 0.075| 11.838| 0.006|2.20e-05| 5.14e-07|\n|WISEA J213454.05-772745.8| 323.72522| -77.46275| G| 372.81| null| null| null| 13.059| 0.143| 13.016| 0.016|4.75e-05| 4.92e-07|\n|WISEA J215146.52-780557.3| 327.94386| -78.09927| G| 368.25| 0.65| null| null| 12.882| 0.135| 12.228| 0.010|2.20e-05| 4.63e-07|\n|WISEA J213733.97-780150.6| 324.39156| -78.03074| G| 345.10| null| 20.443| 0.108| 13.417| 0.204| 11.831| 0.007|1.74e-05| 4.63e-07|\n|WISEA J212943.06-775542.2| 322.42945| -77.92839| G| 372.02| 0.65| null| null| 12.273| 0.114| 12.187| 0.009|1.85e-05| 4.12e-07|\n|WISEA J213608.13-772733.1| 324.03389| -77.45922| G| 319.73| 0.65| 21.351| 0.351| 12.228| 0.076| 11.985| 0.007|2.01e-05| 3.96e-07|\n|WISEA J212913.11-773431.7| 322.30466| -77.57550| G| 378.32| 0.65| 20.656| 0.234| 13.354| 0.165| 12.886| 0.013|2.92e-05| 3.50e-07|\n|WISEA J213641.68-775334.3| 324.17371| -77.89287| G| 411.08| null| 21.445| 0.179| 13.521| 0.179| 13.546| 0.029|4.09e-05| 3.18e-07|\n|WISEA J213632.95-765253.9| 324.13731| -76.88165| G| 460.55| 0.65| null| null| 12.341| 0.134| 12.450| 0.009|1.07e-05| 2.86e-07|\n|WISEA J213724.56-773623.7| 324.35235| -77.60660| G| 536.58| null| null| null| 13.729| 0.212| 13.039| 0.022|1.35e-05| 2.84e-07|\n|WISEA J215903.81-773020.5| 329.76589| -77.50571| G| 426.05| null| null| null| 13.020| 0.144| 12.826| 0.010|1.74e-05| 2.83e-07|\n|WISEA J212501.23-771251.5| 321.25514| -77.21431| G| 436.36| null| null| null| 13.307| 0.143| 12.903| 0.015|1.69e-05| 2.66e-07|\n\nTable 1: Top 20 galaxies in NED-LVS that fall in the 90% probability volume for S250830bp sorted by the joint probability of 3D position and WISE W1 luminosity (P_3D * P_LumW1). Galaxy is the NED preferred name. RA and Dec are the Equatorial coordinates in degrees (J2000). Objtype is the object type of the galaxy candidate. Distance is the distance to the galaxy in Mpc. m_NUV and mErr_NUV are the apparent magnitude and error from GALEX. m_Ks and mErr_Ks are the apparent magnitude and error from 2MASS. m_W1 and mErr_W1 are the apparent magnitude and error from AllWISE. P_3D is the probability that the galaxy is in the volume given the distance of GW event. P_3D_LumW1 is the joint probability within the volume weighted by the WISE1 luminosity of the galaxy (P_3D * P_LumW1). \n",
"published": "2025-08-30T13:50:06.378816Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-30T13:50:06.378830Z",
"modified": "2025-08-30T13:50:06.382816Z",
"targets": []
},
{
"id": 5773940,
"topic": "gcn.circulars",
"uuid": "2f214f4c-5abd-4d08-9e3e-b99abdecb619",
"title": "LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification",
"submitter": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"authors": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41607"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Updated Sky localization, EM Bright Classification, and Source Classification",
"createdOn": 1756558543716,
"submitter": "Colm Talbot at University of Chicago <talbotcolm@gmail.com>",
"circularId": 41607,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250830bp (GCN Circular 41606). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:\n\nhttps://gracedb.ligo.org/superevents/S250830bp\n\nAssuming the candidate is astrophysical, the updated classification of the GW signal, in order of descending probability,\nis BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).\n\nBased on posterior support from parameter estimation [1], under the assumption that the candidate S250830bp is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.\n\nFor the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 4 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):\n icrs; ellipse(21h43m, -77d36m, 1.36d, 0.88d, 15.49d)\nMarginalized over the whole sky, the a posteriori luminosity distance estimate is 427 +/- 69 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040\n [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe",
"published": "2025-08-30T12:55:52.794823Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-30T12:55:52.794837Z",
"modified": "2025-08-30T12:55:52.800248Z",
"targets": []
},
{
"id": 5773897,
"topic": "gcn.circulars",
"uuid": "6eabf084-5fe4-4b4c-b397-76c533d0c21b",
"title": "LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate",
"submitter": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"authors": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41606"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.5.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250830bp",
"subject": "LIGO/Virgo/KAGRA S250830bp: Identification of a GW compact binary merger candidate",
"createdOn": 1756551306846,
"submitter": "Pedro Jeronimo Santos da Silva <pedro.jeronimo@unesp.br>",
"circularId": 41606,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe identified the compact binary merger candidate S250830bp during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-30 10:24:18.852 UTC (GPS time: 1440584676.852). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], MLy [6], PyCBC Live [7], and SPIIR [8] analysis pipelines.\n\nS250830bp is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:\n\nhttps://gracedb.ligo.org/superevents/S250830bp\n\nThe classification of the GW signal, in order of descending probability, is BBH (84%), NSBH (16%), Terrestrial (<1%), or BNS (<1%).\n\nAssuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [9] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [9] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 16%.\n\nThe source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.\n\nTwo sky maps are available at this time and can be retrieved from the GraceDB event page:\n * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [10], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.\n * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [11], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.\n\nThe preferred sky map at this time is bayestar.multiorder.fits,0. For the bayestar.multiorder.fits,0 sky map, the 90% credible region is 11 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 429 +/- 100 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010\n [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004\n [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018\n [4] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008\n [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234\n [6] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034\n [7] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a\n [8] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023\n [9] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe\n [10] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982\n [11] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013\n",
"published": "2025-08-30T10:55:15.436623Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-30T10:55:15.436642Z",
"modified": "2025-08-30T10:55:15.458616Z",
"targets": []
}
]
},
{
"event_id": "S250829ft",
"sequences": [
{
"id": 10063,
"sequence_number": 1,
"sequence_type": "PRELIMINARY",
"message": {
"id": 5773721,
"topic": "igwn.gwalert",
"uuid": "08160456-92b1-4930-a72f-5d089ff04ebf",
"title": "S250829ft - PRELIMINARY",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
"urls": {
"gracedb": "https://gracedb.ligo.org/superevents/S250829ft/view/"
},
"event": {
"far": 2.513998746786633e-06,
"time": "2025-08-29T18:48:20.712Z",
"group": "Burst",
"search": "AllSky",
"area_50": 6389.542520992614,
"area_90": 21821.634775490344,
"duration": 0.21875,
"pipeline": "MLy",
"properties": {},
"instruments": [
"H1",
"L1"
],
"significant": false,
"skymap_hash": "3c471ecc81b434556c758ba5b6eb13dc",
"classification": {},
"skymap_version": 0,
"central_frequency": 56.0
},
"alert_type": "PRELIMINARY",
"sequence_num": 1,
"time_created": "2025-08-29T18:49:11Z",
"superevent_id": "S250829ft",
"external_coinc": null
},
"message_text": "",
"published": "2025-08-29T18:49:12.690483Z",
"message_parser": "IGWN Alert Avro Parser v1",
"retracted": false,
"created": "2025-08-29T18:49:12.690502Z",
"modified": "2025-08-29T18:49:12.693227Z"
}
},
{
"id": 10064,
"sequence_number": 2,
"sequence_type": "PRELIMINARY",
"message": {
"id": 5773724,
"topic": "igwn.gwalert",
"uuid": "d8d5d418-f06a-4a7b-bb04-daed4d062b6c",
"title": "S250829ft - PRELIMINARY",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
"urls": {
"gracedb": "https://gracedb.ligo.org/superevents/S250829ft/view/"
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"event": {
"far": 2.513998746786633e-06,
"time": "2025-08-29T18:48:20.712Z",
"group": "Burst",
"search": "AllSky",
"area_50": 6389.542520992614,
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"duration": 0.21875,
"pipeline": "MLy",
"properties": {},
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],
"significant": false,
"skymap_hash": "3c471ecc81b434556c758ba5b6eb13dc",
"classification": {},
"skymap_version": 0,
"central_frequency": 56.0
},
"alert_type": "PRELIMINARY",
"sequence_num": 2,
"time_created": "2025-08-29T18:53:45Z",
"superevent_id": "S250829ft",
"external_coinc": null
},
"message_text": "",
"published": "2025-08-29T18:53:46.455627Z",
"message_parser": "IGWN Alert Avro Parser v1",
"retracted": false,
"created": "2025-08-29T18:53:46.455654Z",
"modified": "2025-08-29T18:53:46.458898Z"
}
}
],
"references": []
},
{
"event_id": "S250829gn",
"sequences": [
{
"id": 10065,
"sequence_number": 1,
"sequence_type": "PRELIMINARY",
"message": {
"id": 5773774,
"topic": "igwn.gwalert",
"uuid": "421f19a0-34cd-4a4b-aeb1-b89e9414422d",
"title": "S250829gn - PRELIMINARY",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
"urls": {
"skymap": "https://gracedb.ligo.org/api/superevents/S250829gn/files/bayestar.multiorder.fits,0",
"gracedb": "https://gracedb.ligo.org/superevents/S250829gn/view/"
},
"event": {
"far": 7.370559585127547e-06,
"time": "2025-08-29T23:20:29.493Z",
"group": "CBC",
"search": "AllSky",
"area_50": 1156.9662899235936,
"area_90": 4212.414805314847,
"duration": null,
"pipeline": "gstlal",
"properties": {
"HasNS": 0.0,
"HasMassGap": 0.005275362318840579,
"HasRemnant": 0.0
},
"instruments": [
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"L1"
],
"significant": false,
"skymap_hash": "f450174a8a7e85cc949dbf583512b321",
"classification": {
"BBH": 0.16746960989245288,
"BNS": 3.1247108365590026e-15,
"NSBH": 2.225232437984007e-13,
"Terrestrial": 0.8325303901073214
},
"skymap_version": 0,
"central_frequency": null
},
"alert_type": "PRELIMINARY",
"sequence_num": 1,
"time_created": "2025-08-29T23:20:51Z",
"superevent_id": "S250829gn",
"external_coinc": null
},
"message_text": "",
"published": "2025-08-29T23:20:53.246652Z",
"message_parser": "IGWN Alert Avro Parser v1",
"retracted": false,
"created": "2025-08-29T23:20:53.246678Z",
"modified": "2025-08-29T23:20:53.264034Z"
}
},
{
"id": 10066,
"sequence_number": 2,
"sequence_type": "PRELIMINARY",
"message": {
"id": 5773775,
"topic": "igwn.gwalert",
"uuid": "31cd65de-d72d-4761-9e9c-23ac2c899039",
"title": "S250829gn - PRELIMINARY",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
"urls": {
"skymap": "https://gracedb.ligo.org/api/superevents/S250829gn/files/bayestar.multiorder.fits,1",
"gracedb": "https://gracedb.ligo.org/superevents/S250829gn/view/"
},
"event": {
"far": 7.370559585127547e-06,
"time": "2025-08-29T23:20:29.492Z",
"group": "CBC",
"search": "AllSky",
"area_50": 1139.1312999628565,
"area_90": 4129.324734438942,
"duration": null,
"pipeline": "gstlal",
"properties": {
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"HasMassGap": 0.005275362318840579,
"HasRemnant": 0.0
},
"instruments": [
"H1",
"L1"
],
"significant": false,
"skymap_hash": "1c7f975b06101264404c48cbf66c98ff",
"classification": {
"BBH": 0.16746960989245288,
"BNS": 3.1247108365590026e-15,
"NSBH": 2.225232437984007e-13,
"Terrestrial": 0.8325303901073214
},
"skymap_version": 1,
"central_frequency": null
},
"alert_type": "PRELIMINARY",
"sequence_num": 2,
"time_created": "2025-08-29T23:25:26Z",
"superevent_id": "S250829gn",
"external_coinc": null
},
"message_text": "",
"published": "2025-08-29T23:25:28.030522Z",
"message_parser": "IGWN Alert Avro Parser v1",
"retracted": false,
"created": "2025-08-29T23:25:28.030542Z",
"modified": "2025-08-29T23:25:28.042380Z"
}
}
],
"references": []
},
{
"event_id": "S250822bx",
"sequences": [
{
"id": 9854,
"sequence_number": 1,
"sequence_type": "PRELIMINARY",
"message": {
"id": 5771881,
"topic": "igwn.gwalert",
"uuid": "74873ba2-68be-4495-9e9f-324a1cb3cf09",
"title": "S250822bx - PRELIMINARY",
"submitter": "cody.messick-30d512c8",
"authors": "LVK",
"data": {
"urls": {
"skymap": "https://gracedb.ligo.org/api/superevents/S250822bx/files/cwb.multiorder.fits,0",
"gracedb": "https://gracedb.ligo.org/superevents/S250822bx/view/"
},
"event": {
"far": 5.86007e-06,
"time": "2025-08-22T19:24:56.235Z",
"group": "Burst",
"search": "AllSky",
"area_50": 819.9898913713101,
"area_90": 3107.0650745717408,
"duration": 0.017009,
"pipeline": "CWB",
"properties": {},
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],
"significant": false,
"skymap_hash": "1394646eb2c163e381b4b573a140a847",
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{
"id": 5772526,
"topic": "gcn.circulars",
"uuid": "a8b8542c-6b70-40b1-8464-a603a8ebd578",
"title": "LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD observations of AT2025smm",
"submitter": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
"authors": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
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"bibcode": "2025GCN.41216....1C",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD observations of AT2025smm",
"createdOn": 1754088479192,
"submitter": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
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"submittedHow": "web"
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"message_text": "Wagner Corradi (LNA), Nélio Sasaki (UEA-Parintins), Leandro de Almeida (LNA), Felipe Navarete (LNA), S. Antier (OCA/IJCLAB), Yodgor Rajabov (UBAI), Yusufjion Tillayev (UBAI), Eslam Elhosseiny (NRIAG), C. Andrade (UMN), D. Turpin (CEA), I. Tosta e Melo (IFN), M. Coughlin (UMN), P. Hello (IJCLAB), P-A. Duverne (APC), S. Karpov (FZU) and T. Pradier (IPHC) on behalf of Observatório do Pico dos Dias (OPD/LNA/Brazil) and the GRANDMA collaboration, we inspected the optical candidate AT2025smm (GCNs 41179) of the gravitational-wave event S250727dc (GCNs 41179, 41487), reported by Swift/UVOT (GCN 41179,GCN 41187, GCN 41188, GCN 41196, GCN 41197, GCN 41201, GCN 41202, GCN 41205, GCN 41211 and GCN 41214). Observations were conducted with the 0.6 m telescope at OPD, obtaining six frames of 300s exposures in the Bessel-R band between 2025-07-31T08:18:35.268 and 2025-07-31T08:48:35.268 UTC. No source is detected at the reported position in the stacked image. The final stacked frame covers a field of view of 11.6′ × 11.6′ with a pixel scale of 0.34″ arcsecond per pixel.\nHere we report the following limiting magnitudes, uncorrected for Galactic extinction:\nFilter | Mag (AB)\n r | > 22.53\nThis result is in agreement with the analysis of DECam GW-MMADS (GCN 41197).\nWe use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (http://grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518).",
"published": "2025-08-25T08:08:46.345123Z",
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"created": "2025-08-25T08:08:46.345138Z",
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{
"id": 3073785,
"topic": "gcn.circulars",
"uuid": "82c22a23-8c08-4c5c-aeda-5cb47abea450",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates",
"submitter": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
"authors": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
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"bibcode": "2025GCN.41214....1C",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates",
"createdOn": 1753993273687,
"submitter": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
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"message_text": "Tomás Cabrera (CMU), Xander J. Hall (CMU), Lei Hu (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team\n\nDECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and visually inspect the remaining transients. All candidates included in this GCN have been reported to the TNS. We report the novel candidates that pass our cuts at this stage in the first table; in the following sections we present genuine transients we reject as gravitational wave counterparts, and the confirmation of some previously reported candidates.\n\nWe report the discovery of the following 3 nuclear transients that we do not exclude from our search. For AT2025stf, the nucleus of the host is ambiguous, and so it is possible this transient is non-nuclear, which would eliminate it as a favored candidate.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290634429m374411 | AT 2025ssz | 98.678750 | -37.736389 | 2025-07-29 09:49:00.192 | 21.476 | 0.140 | 0.487 |\n| T202507290636497m363725 | AT 2025ste | 99.207083 | -36.623611 | 2025-07-29 09:49:00.192 | 22.437 | 0.408 | 1.627 |\n| T202507290626025m372843 | AT 2025stf | 96.510373 | -37.478478 | 2025-07-29 09:47:11.817 | 21.193 | 0.073 | -0.211 |\n\nThe following transients are coincident with objects in the quaia and Milliquas catalogs, and have redshifts that place them outside of the 90% localization volume for the gravitational wave event.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | possible_host | host_z |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | ----------- | --------- |\n| T202507290621597m351731 | AT 2025ssx | 95.498727 | -35.291956 | 2025-07-29 10:26:17.504 | 20.945 | 0.061 | 0.661 | quaia 2891332298170436480 | 0.47 |\n| T202507290634416m350638 | AT 2025sta | 98.673141 | -35.110434 | 2025-07-29 09:52:36.137 | 19.783 | 0.047 | 0.288 | quaia 5581689916779289344 | 0.53 |\n| T202507290629125m373923 | AT 2025std | 97.302216 | -37.656276 | 2025-07-29 09:49:00.617 | 19.945 | 0.035 | 0.819 | milliquas 6dF J062912.5-373922 | 0.06 |\n\nThe following transients are nuclear, but are hosted by nearby galaxies (as determined by their large angular sizes), and so are unlikely to be associated with the gravitational wave event.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290623386m353830 | AT 2025ssy | 95.910743 | -35.641755 | 2025-07-29 09:45:22.413 | 19.100 | 0.013 | 0.454 |\n| T202507290627447m354045 | AT 2025stg | 96.936244 | -35.679174 | 2025-07-29 09:58:01.892 | 20.724 | 0.047 | 0.916 |\n| T202507290618452m351815 | AT 2025sth | 94.688504 | -35.304203 | 2025-07-29 09:45:22.413 | 19.080 | 0.014 | 0.441 |\n| T202507290627067m352915 | AT 2025sti | 96.778033 | -35.487632 | 2025-07-29 09:58:01.892 | 17.192 | 0.004 | 0.622 |\n\nThe following transients are non-nuclear, and appear to be supernovae.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290626444m373046 | AT 2025stb | 96.684801 | -37.512899 | 2025-07-29 09:47:11.817 | 21.174 | 0.072 | 0.028 |\n| T202507290625106m332028 | AT 2025stc | 96.294042 | -33.341084 | 2025-07-29 09:56:12.517 | 20.252 | 0.056 | -0.166 |\n \nWe additionally confirm the following candidates initially reported in GCN 41211, including the nuclear candidate with a Quaia host at a favorable redshift (AT 2025ssi).\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290636054m360136 | AT 2025ssf | 99.022404 | -36.026624 | 2025-07-29 10:23:00.741 | 20.624 | 0.040 | [no mag_i] |\n| T202507290631153m360431 | AT 2025ssg | 97.813758 | -36.075317 | 2025-07-29 10:23:00.741 | 20.412 | 0.028 | [no mag_i] |\n| T202507290636352m372956 | AT 2025ssi | 99.146615 | -37.499005 | 2025-07-29 09:49:00.617 | 20.112 | 0.047 | 0.391 |\n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
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{
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"uuid": "44e1bc1e-ad4a-44a3-a01b-af32b1361e97",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm",
"submitter": "xjh@andrew.cmu.edu",
"authors": "xjh@andrew.cmu.edu",
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"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41197....1H",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm",
"createdOn": 1753822532121,
"submitter": "xjh@andrew.cmu.edu",
"circularId": 41197,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team\n\nDECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).\n\nWe inspected the location of AT 2025smm, reported by Swift UVOT (GCN 41187), we find no sources in the ugri-bands. Here we report the following limiting magnitudes, not corrected for Galactic extinction:\n\n| Filter | Mag (AB) |\n|--------|----------|\n| g | > 22.15 |\n| r | > 21.46 | \n| i | > 20.60 | \n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-08-04T08:08:58.223382Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:58.223394Z",
"modified": "2025-08-04T08:08:58.226840Z",
"targets": []
},
{
"id": 3073769,
"topic": "gcn.circulars",
"uuid": "28d35eac-0c2c-4976-beb5-f6aa92fb8939",
"title": "LIGO/Virgo/KAGRA S250727dc: Identification of a GW compact binary merger candidate",
"submitter": "aubrey_laity@uri.edu",
"authors": "aubrey_laity@uri.edu",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41179"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41179....1L",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Identification of a GW compact binary merger candidate",
"createdOn": 1753647898730,
"submitter": "aubrey_laity@uri.edu",
"circularId": 41179,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe identified the compact binary merger candidate S250727dc during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-07-27 20:01:48.457 UTC (GPS time: 1437681726.457). The candidate was found by the cWB BBH [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.\n\nS250727dc is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.1e-18 Hz, or about one in 1e10 years. The event's properties can be found at this URL:\n\nhttps://gracedb.ligo.org/superevents/S250727dc\n\nThe classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%).\n\nAssuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.\n\nThe source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.\n\nTwo sky maps are available at this time and can be retrieved from the GraceDB event page:\n * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 24 seconds after the candidate event time.\n * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.\n\nThe preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is well fit by an ellipse with an area of 23 deg^2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):\n icrs; ellipse(06h27m, -36d17m, 3.74d, 1.98d, 29.70d)\nMarginalized over the whole sky, the a posteriori luminosity distance estimate is 1172 +/- 256 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018\n [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008\n [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234\n [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a\n [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023\n [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe\n [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013",
"published": "2025-08-04T08:08:58.182500Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:58.182513Z",
"modified": "2025-08-04T08:08:58.185998Z",
"targets": []
},
{
"id": 3073761,
"topic": "gcn.circulars",
"uuid": "970b759e-c5a9-4c0c-8ee5-af3b9b562a57",
"title": "LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO",
"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/41202"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41202....1W",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO",
"createdOn": 1753897772173,
"submitter": "Maia Williams at PSU <mjw6837@psu.edu>",
"circularId": 41202,
"submittedHow": "web"
},
"message_text": "Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:\n\nSwift/BAT was observing 100.00% of the GW localization probability ([Bilby.offline0.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250727dc/files/Bilby.offline0.multiorder.fits)) at merger time. A fraction 100.00% of the GW localization posterior is contained inside the BAT coded FoV.\n\nThe LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).\n\nUpon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.\n\nUsing the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)) we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.\n\nWe quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.\nIn units of 10^-7 erg/s/cm^2:\n\n|time_bin (s) |soft |normal|hard |GRB170817\n|-|-|-|-|-|\n|0.256 |0.56 |0.87 |0.79 |0.92 \n|1.024 |0.28 |0.44 |0.40 |0.47 \n|4.096 |0.15 |0.24 |0.22 |0.25 \n|16.384 |0.10 |0.15 |0.14 |0.16 \n\n\nThe upper limits as function of sky position are plotted here, alongside the GW localization: https://zenodo.org/records/16612515\n\nThe solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.\n\nGUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.\n\nA live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/",
"published": "2025-08-04T08:08:57.093110Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:57.093128Z",
"modified": "2025-08-04T08:08:57.096671Z",
"targets": []
},
{
"id": 3073752,
"topic": "gcn.circulars",
"uuid": "ba10240b-d0e8-4f76-84c2-cc15859eb647",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41187"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41187....1K",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"version": 3,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>",
"editedOn": 1753971347301,
"createdOn": 1753716685802,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41187,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift observed the high-probability region of LIGO/Virgo/KAGRA S250725dc\nstarting 65.3 minutes after the trigger date. A new UVOT source was found\nin the U band with U = 19.9 +/- 0.3 mag (ABmag) image taken on 2025-07-28\n00:34_55 UT at the following position:\n\nRA = 97.47619, Dec = -37.23307 deg, or RA = 06:29:54.3,\nDec = -37:13:55.45 in sexagesimal notation.\n\nWe do not find any source on that location in DSS, using the Vizier\nphotometry tool, or as a nearby object known to the Minor Planet Center\nMPChecker tool. The object has been registered as 2025smm by the Transient\nName Server.\n\nA further Swift observation has been planned.\n",
"published": "2025-08-04T08:08:56.014318Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:56.014331Z",
"modified": "2025-08-04T08:08:56.019984Z",
"targets": []
},
{
"id": 3073741,
"topic": "gcn.circulars",
"uuid": "8e688316-f2f5-45f9-826a-33d0838a9454",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources",
"submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"authors": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41201"
},
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41201....1E",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources",
"createdOn": 1753872329291,
"submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"circularId": 41201,
"submittedHow": "email"
},
"message_text": "P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko\n(NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P.\nBeardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S.\nCampana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina), S.\nDichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia\n(ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N.\nKlingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha\n(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien\n(U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato\n(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),\nE. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:\n\nSwift has carried out 171 observations of the LVK error region for the GW trigger\nS250727dc convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),\nusing the 'bayestar.multiorder.fits,1' GW localisation map. As this is a 3D skymap,\ngalaxy distances were taken into account in selecting which ones to observe. The\nobservations currently span from 3.9 ks to 157 ks after the LVK trigger, and the XRT\nhas covered 11.5 deg^2 on the sky (corrected for overlaps). This covers 70% of the\nprobability in the imrpoved 'Bilby.offline0.multiorder.fits' skymap, and 69% after\nconvolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016,\nMNRAS,\t462, 1591).\n\nIn total, we have detected 8 X-ray sources. Each source is assigned a rank of 1-4\nwhich describes how likely it is to be related to the GW trigger, with 1 being the\nmost likely and 4 being the least likely. The ranks are described at\nhttps://www.swift.ac.uk/ranks.php.\n\nWe have found:\n\n * 0 sources of rank 1\n * 0 sources of rank 2 (see note)\n * 3 sources of rank 3\n * 5 sources of rank 4\n\nNOTE: The online system reports a rank 2 source: Source S250727dc_X1. However, this\nin the known blazar BL Lac (the prototype of its class of object). During our\nobservation, its flux was significantly increased compared to the mean value in the\nLSXPS catalogue, hence the automated classification. However, it is known to be\nhighly variable (e.g.\nhttps://www.swift.psu.edu/monitoring/source.php?source=BLLacertae) and the\nintensities in LSXPS are drawn solely from PC-mode data so are biased towards the\nlower-intensity states. We have thus reclassified this as rank 4 for this circular.\n\n\nRANK 3 sources\n==============\n\nThese are uncatalogued X-ray sources, however they are not brighter than previous\nupper limits, so do not stand out as likely counterparts to the GW trigger.\n\n| Source ID\t | RA\t\t | Dec \t | Err90 | Detection Flag\t|\n| S250727dc_X5 | 06h 23m 38.04s | -35d 38' 24.1\" |\t7.7\" |\t\t GOOD |\n| S250727dc_X6 | 06h 28m 07.71s | -37d 10' 10.3\" |\t9.5\" |\t REASONABLE |\n| S250727dc_X7 | 06h 18m 45.03s | -35d 18' 15.1\" |\t7.0\" |\t\t GOOD |\n\n\nRANK 4 sources\n==============\n\nThese are catalogued X-ray sources, showing no signs of outburst compared to\nprevious observations, so they are not likely to be related to the GW trigger.\n\n| Source ID\t | RA\t\t | Dec \t | Err90 | Detection Flag\t|\n| S250727dc_X1 | 06h 27m 06.55s | -35d 29' 15.0\" |\t3.8\" |\t\t GOOD |\n| S250727dc_X3 | 06h 34m 35.33s | -37d 17' 12.1\" |\t8.3\" |\t\t GOOD |\n| S250727dc_X8 | 06h 36m 18.37s | -37d 22' 33.4\" |\t4.5\" |\t\t GOOD |\n| S250727dc_X9 | 06h 19m 53.43s | -35d 24' 28.2\" |\t7.7\" |\t\t GOOD |\n| S250727dc_X10 | 06h 20m 08.64s | -34d 37' 54.4\" |\t5.8\" |\t\t GOOD |\n\n\nIn addition, we took a 1.9 ks exposure of the UVOT source reported by Kuin et al.\n(GCN Circ. 41187). No XRT counterpart was found, with a 3-sigma upper limit of\n3.9e-3 ct/sec (approx 1.6e-13 erg cm^-s s^-1, 0.3-10 keV).\n\nThe results of the XRT automated analysis, including details of the sources listed\nabove, are online at https://www.swift.ac.uk/LVC/S250727dc\n\nThis circular is an official product of the Swift XRT team.\n\n\n",
"published": "2025-08-04T08:08:52.906958Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:52.906970Z",
"modified": "2025-08-04T08:08:52.910503Z",
"targets": []
},
{
"id": 3073738,
"topic": "gcn.circulars",
"uuid": "e20ae96b-9d78-4a89-9f04-85895c7a7348",
"title": "LIGO/Virgo/KAGRA S250727dc: Updated Sky localization",
"submitter": "lucy.thomas@ligo.org",
"authors": "lucy.thomas@ligo.org",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41188"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41188....1L",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Updated Sky localization",
"createdOn": 1753724455424,
"submitter": "lucy.thomas@ligo.org",
"circularId": 41188,
"submittedHow": "web"
},
"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250727dc (GCN Circular 41179). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:\n\nhttps://gracedb.ligo.org/superevents/S250727dc\n\nFor the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 15 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):\n icrs; ellipse(06h29m, -35d45m, 2.97d, 1.66d, 27.69d)\nMarginalized over the whole sky, the a posteriori luminosity distance estimate is 1389 +/- 318 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040",
"published": "2025-08-04T08:08:52.861375Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:52.861387Z",
"modified": "2025-08-04T08:08:52.864829Z",
"targets": []
},
{
"id": 3073728,
"topic": "gcn.circulars",
"uuid": "399ea105-fc2a-41b5-ac25-2fd03da7e0f7",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41205"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41205....1K",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"version": 2,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>",
"editedOn": 1753971544469,
"createdOn": 1753907305327,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41205,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift re-observed the location of AT 2025smm in the high-probability region\nof LIGO/Virgo/KAGRA S250725dc from 1.56 to 1.63 d after the trigger. The\nsource is no longer detected. we obtained the following 3-sigma upper\nlimits:\n\nFilter Exposure(s) 3-sigma UL AB mag)\nUVW2 606 22.3\nUVM2 466 21.8\nUVW1 303 21.5\n U 151 20.8\n B 151 20.1\n V 151 19.3\n\nNo corrections were made for interstellar extinction.\n",
"published": "2025-08-04T08:08:52.749607Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:52.749620Z",
"modified": "2025-08-04T08:08:52.753238Z",
"targets": []
},
{
"id": 3073720,
"topic": "gcn.circulars",
"uuid": "2de35f71-61c0-48f5-b8af-78c6cb3916f0",
"title": "LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm",
"submitter": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"authors": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41196"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41196....1D",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm",
"createdOn": 1753812667976,
"submitter": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"circularId": 41196,
"submittedHow": "web"
},
"message_text": "LIGO/Virgo/KAGRA S250727dc : PRIME J-band upper limits of AT 2025smm\n\nJ. Durbak (UMD), O. Guiffreda (UMD), N. Passaleva, M. El Kabir, A. S. Kutyrev (NASA/GSFC), E. Troja (U Rome), S. B. Cenko (NASA/GSFC)\n\nFollowing the LIGO/Virgo/KAGRA detection of S250727dc (GCN 41179), we observed a portion of the transient field in J-filter with PRIME ~32 hours after LVK detection. This field was chosen to overlap with the Swift UVOT detection of an uncatalogued source, AT 2025smm (GCN 41187).\n\nAt the position of AT 2025smm reported by Swift UVOT (GCN 41187), we detect no sources in J-band. Using nearby 2MASS stars for preliminary calibration we derive the following limiting magnitude, not corrected for Galactic extinction: \n\n| Filter | Mag(AB) |\n|--------|--------------|\n| J | > 19.7 | \n\nPRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).\n\nWe thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.",
"published": "2025-08-04T08:08:51.660207Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:51.660219Z",
"modified": "2025-08-04T08:08:51.663496Z",
"targets": []
},
{
"id": 3073710,
"topic": "gcn.circulars",
"uuid": "9c48891d-93d6-48af-b910-bc3690b33aee",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41211"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"bibcode": "2025GCN.41211....1M",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates",
"createdOn": 1753967951880,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41211,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, H. Thomas Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-07-29 09:39:10 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250727dc (GCN 41179). Observations covered the 90% localization region of the event (Prop ID: 2023B-851374; PI Andreoni/Palmese).\n\nWe process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. We require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al. 2015, Flesch 2023, Storey-Fisher et al. 2024, Souchay et al. 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.\n\nAfter candidate selection we report the 4 high confidence candidates listed below, including 2 nuclear candidates (likely AGN), and 1 likely supernova. Additionally, the transient AT2025ssi is the probable AGN Quaia_1070591 with a measured redshift of z~0.24 +- 0.08, which falls within the reported LVK volume. Further observations are ongoing, and we encourage followup of the 4 candidates identified herein.\n\n| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|\n| SN_LIKE | 3334740 | AT2025ssf | 99.02238 | -36.026584 | 20.65 | 0.05 | 20.32 | 0.03 | N/A | N/A | 21.21 | 0.12 |\n| AGN_LIKE | 3334614 | AT2025ssg | 97.813775 | -36.075323 | 21.06 | 0.1 | 20.58 | 0.07 | N/A | N/A | 19.99 | 0.04 |\n| AGN_LIKE | 3334106 | AT2025ssh | 97.339644 | -36.600508 | 21.79 | 0.21 | N/A | N/A | 21.08 | 0.09 | 19.5 | 0.04 |\n| AGN_LIKE | 3334650 | AT2025ssi | 99.146548 | -37.499065 | 20.09 | 0.05 | N/A | N/A | 19.68 | 0.03 | N/A | N/A |\n\nAdditionally, we also recover two of the candidates reported by SWIFT-XRT (GCN 41201). One is the known blazar BL Lacertae (S250727dc_X1), which has been mentioned to be highly variable. The other (S250727dc_X7) is the galaxy ESO 365-5. Both of these galaxies have been spectroscopically measured to have redshifts z=0.055 (García-Pérez et al. 2024) and z=0.045 (Jones et al. 2009) respectively, which both lie outside of the LVK localization volume.\n\n| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|\n| AGN_LIKE | 3334338 | S250727dc_X1 | 96.778098 | -35.487631 | 17.32 | 0.01 | 16.98 | 0.01 | N/A | N/A | 16.5 | 0.0 |\n| AGN_LIKE | 3334206 | S250727dc_X7 | 94.688512 | -35.304098 | 18.79 | 0.02 | 18.32 | 0.02 | 18.34 | 0.01 | 18.51 | 0.02 |\n\nWe do not recover any of the other candidates reported by SWIFT-XRT, nor the transient AT2025smm reported by SWIFT-UVOT (GCN 41187).\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-08-04T08:08:50.547495Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-04T08:08:50.547508Z",
"modified": "2025-08-04T08:08:50.551187Z",
"targets": []
},
{
"id": 2585540,
"topic": "gcn.circulars",
"uuid": "379e0d19-8a43-4f06-b311-74650ae65833",
"title": "LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD observations of AT2025smm",
"submitter": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
"authors": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41216"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: GRANDMA/OPD observations of AT2025smm",
"createdOn": 1754088479192,
"submitter": "Nelio Sasaki at NEPA-UEA <nsasaki@uea.edu.br>",
"circularId": 41216,
"submittedHow": "web"
},
"message_text": "Wagner Corradi (LNA), Nélio Sasaki (UEA-Parintins), Leandro de Almeida (LNA), Felipe Navarete (LNA), S. Antier (OCA/IJCLAB), Yodgor Rajabov (UBAI), Yusufjion Tillayev (UBAI), Eslam Elhosseiny (NRIAG), C. Andrade (UMN), D. Turpin (CEA), I. Tosta e Melo (IFN), M. Coughlin (UMN), P. Hello (IJCLAB), P-A. Duverne (APC), S. Karpov (FZU) and T. Pradier (IPHC) on behalf of Observatório do Pico dos Dias (OPD/LNA/Brazil) and the GRANDMA collaboration, we inspected the optical candidate AT2025smm (GCNs 41179) of the gravitational-wave event S250727dc (GCNs 41179, 41487), reported by Swift/UVOT (GCN 41179,GCN 41187, GCN 41188, GCN 41196, GCN 41197, GCN 41201, GCN 41202, GCN 41205, GCN 41211 and GCN 41214). Observations were conducted with the 0.6 m telescope at OPD, obtaining six frames of 300s exposures in the Bessel-R band between 2025-07-31T08:18:35.268 and 2025-07-31T08:48:35.268 UTC. No source is detected at the reported position in the stacked image. The final stacked frame covers a field of view of 11.6′ × 11.6′ with a pixel scale of 0.34″ arcsecond per pixel.\nHere we report the following limiting magnitudes, uncorrected for Galactic extinction:\nFilter | Mag (AB)\n r | > 22.53\nThis result is in agreement with the analysis of DECam GW-MMADS (GCN 41197).\nWe use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (http://grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518).",
"published": "2025-08-01T22:48:06.599854Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-08-01T22:48:06.599872Z",
"modified": "2025-08-01T22:48:06.605457Z",
"targets": []
},
{
"id": 2385869,
"topic": "gcn.circulars",
"uuid": "75fee4f7-5cc7-493f-a051-8dc4f49b21d0",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates",
"submitter": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
"authors": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41214"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS candidates",
"createdOn": 1753993273687,
"submitter": "Tomas Cabrera at Carnegie Mellon University <tcabrera@andrew.cmu.edu>",
"circularId": 41214,
"submittedHow": "web"
},
"message_text": "Tomás Cabrera (CMU), Xander J. Hall (CMU), Lei Hu (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team\n\nDECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).\n\nWe run the SFFT difference imaging (Hu et al. 2022) on the available images, filter out likely stars and moving objects, and visually inspect the remaining transients. All candidates included in this GCN have been reported to the TNS. We report the novel candidates that pass our cuts at this stage in the first table; in the following sections we present genuine transients we reject as gravitational wave counterparts, and the confirmation of some previously reported candidates.\n\nWe report the discovery of the following 3 nuclear transients that we do not exclude from our search. For AT2025stf, the nucleus of the host is ambiguous, and so it is possible this transient is non-nuclear, which would eliminate it as a favored candidate.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290634429m374411 | AT 2025ssz | 98.678750 | -37.736389 | 2025-07-29 09:49:00.192 | 21.476 | 0.140 | 0.487 |\n| T202507290636497m363725 | AT 2025ste | 99.207083 | -36.623611 | 2025-07-29 09:49:00.192 | 22.437 | 0.408 | 1.627 |\n| T202507290626025m372843 | AT 2025stf | 96.510373 | -37.478478 | 2025-07-29 09:47:11.817 | 21.193 | 0.073 | -0.211 |\n\nThe following transients are coincident with objects in the quaia and Milliquas catalogs, and have redshifts that place them outside of the 90% localization volume for the gravitational wave event.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i | possible_host | host_z |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- | ----------- | --------- |\n| T202507290621597m351731 | AT 2025ssx | 95.498727 | -35.291956 | 2025-07-29 10:26:17.504 | 20.945 | 0.061 | 0.661 | quaia 2891332298170436480 | 0.47 |\n| T202507290634416m350638 | AT 2025sta | 98.673141 | -35.110434 | 2025-07-29 09:52:36.137 | 19.783 | 0.047 | 0.288 | quaia 5581689916779289344 | 0.53 |\n| T202507290629125m373923 | AT 2025std | 97.302216 | -37.656276 | 2025-07-29 09:49:00.617 | 19.945 | 0.035 | 0.819 | milliquas 6dF J062912.5-373922 | 0.06 |\n\nThe following transients are nuclear, but are hosted by nearby galaxies (as determined by their large angular sizes), and so are unlikely to be associated with the gravitational wave event.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290623386m353830 | AT 2025ssy | 95.910743 | -35.641755 | 2025-07-29 09:45:22.413 | 19.100 | 0.013 | 0.454 |\n| T202507290627447m354045 | AT 2025stg | 96.936244 | -35.679174 | 2025-07-29 09:58:01.892 | 20.724 | 0.047 | 0.916 |\n| T202507290618452m351815 | AT 2025sth | 94.688504 | -35.304203 | 2025-07-29 09:45:22.413 | 19.080 | 0.014 | 0.441 |\n| T202507290627067m352915 | AT 2025sti | 96.778033 | -35.487632 | 2025-07-29 09:58:01.892 | 17.192 | 0.004 | 0.622 |\n\nThe following transients are non-nuclear, and appear to be supernovae.\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290626444m373046 | AT 2025stb | 96.684801 | -37.512899 | 2025-07-29 09:47:11.817 | 21.174 | 0.072 | 0.028 |\n| T202507290625106m332028 | AT 2025stc | 96.294042 | -33.341084 | 2025-07-29 09:56:12.517 | 20.252 | 0.056 | -0.166 |\n \nWe additionally confirm the following candidates initially reported in GCN 41211, including the nuclear candidate with a Quaia host at a favorable redshift (AT 2025ssi).\n\n| internal_id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_g-mag_i |\n| -------------- | ------- | -- | --- | ------------------- | ----- | --------- | ----------- |\n| T202507290636054m360136 | AT 2025ssf | 99.022404 | -36.026624 | 2025-07-29 10:23:00.741 | 20.624 | 0.040 | [no mag_i] |\n| T202507290631153m360431 | AT 2025ssg | 97.813758 | -36.075317 | 2025-07-29 10:23:00.741 | 20.412 | 0.028 | [no mag_i] |\n| T202507290636352m372956 | AT 2025ssi | 99.146615 | -37.499005 | 2025-07-29 09:49:00.617 | 20.112 | 0.047 | 0.391 |\n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-07-31T20:21:20.914013Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-31T20:21:20.914030Z",
"modified": "2025-07-31T20:21:20.926668Z",
"targets": []
},
{
"id": 2369505,
"topic": "gcn.circulars",
"uuid": "cba25007-c0d7-4322-8c05-f6c9fe4cbc42",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41205"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"version": 2,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>",
"editedOn": 1753971544469,
"createdOn": 1753907305327,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41205,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift re-observed the location of AT 2025smm in the high-probability region\nof LIGO/Virgo/KAGRA S250725dc from 1.56 to 1.63 d after the trigger. The\nsource is no longer detected. we obtained the following 3-sigma upper\nlimits:\n\nFilter Exposure(s) 3-sigma UL AB mag)\nUVW2 606 22.3\nUVM2 466 21.8\nUVW1 303 21.5\n U 151 20.8\n B 151 20.1\n V 151 19.3\n\nNo corrections were made for interstellar extinction.\n",
"published": "2025-07-31T14:19:08.203662Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-31T14:19:08.203678Z",
"modified": "2025-07-31T14:19:08.209088Z",
"targets": []
},
{
"id": 2368899,
"topic": "gcn.circulars",
"uuid": "1f783d2c-efdf-4574-bbf1-4f83f1bbedd7",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41187"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"version": 3,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>",
"editedOn": 1753971347301,
"createdOn": 1753716685802,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41187,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DAWN/NBI and Radboud), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift observed the high-probability region of LIGO/Virgo/KAGRA S250725dc\nstarting 65.3 minutes after the trigger date. A new UVOT source was found\nin the U band with U = 19.9 +/- 0.3 mag (ABmag) image taken on 2025-07-28\n00:34_55 UT at the following position:\n\nRA = 97.47619, Dec = -37.23307 deg, or RA = 06:29:54.3,\nDec = -37:13:55.45 in sexagesimal notation.\n\nWe do not find any source on that location in DSS, using the Vizier\nphotometry tool, or as a nearby object known to the Minor Planet Center\nMPChecker tool. The object has been registered as 2025smm by the Transient\nName Server.\n\nA further Swift observation has been planned.\n",
"published": "2025-07-31T14:15:51.125828Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-31T14:15:51.125843Z",
"modified": "2025-07-31T14:15:51.131580Z",
"targets": []
},
{
"id": 2358466,
"topic": "gcn.circulars",
"uuid": "f6aba1f0-4601-4cb9-a0c9-efd94af580cd",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates",
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"authors": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41211"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam DESGW Epoch 1 Candidates",
"createdOn": 1753967951880,
"submitter": "Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>",
"circularId": 41211,
"submittedHow": "web"
},
"message_text": "Isaac McMahon, Sean MacBride, Marcelle Soares-Santos (UZH), Nora Sherman (Boston U.), Simran Kaur (U. of Michigan/UZH), Lillian Joseph (Benedictine U.), Ken Herner, H. Thomas Diehl (Fermilab), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:\n\nAt 2025-07-29 09:39:10 UTC, the Dark Energy Camera (DECam) began the first epoch of observations in response to the LVK alert issued for the candidate gravitational-wave event S250727dc (GCN 41179). Observations covered the 90% localization region of the event (Prop ID: 2023B-851374; PI Andreoni/Palmese).\n\nWe process the images with our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al 2015) to reject subtraction artifacts. We require an autoscan score of at least 0.7 on at least one of those detections. We also match our candidates against the ALLWISE, Milliquas, Quaia, and LQAC-6 AGN catalogs (Secrest et al. 2015, Flesch 2023, Storey-Fisher et al. 2024, Souchay et al. 2024) within the LVK localization volume to determine if any correspond to known active galactic nuclei.\n\nAfter candidate selection we report the 4 high confidence candidates listed below, including 2 nuclear candidates (likely AGN), and 1 likely supernova. Additionally, the transient AT2025ssi is the probable AGN Quaia_1070591 with a measured redshift of z~0.24 +- 0.08, which falls within the reported LVK volume. Further observations are ongoing, and we encourage followup of the 4 candidates identified herein.\n\n| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|\n| SN_LIKE | 3334740 | AT2025ssf | 99.02238 | -36.026584 | 20.65 | 0.05 | 20.32 | 0.03 | N/A | N/A | 21.21 | 0.12 |\n| AGN_LIKE | 3334614 | AT2025ssg | 97.813775 | -36.075323 | 21.06 | 0.1 | 20.58 | 0.07 | N/A | N/A | 19.99 | 0.04 |\n| AGN_LIKE | 3334106 | AT2025ssh | 97.339644 | -36.600508 | 21.79 | 0.21 | N/A | N/A | 21.08 | 0.09 | 19.5 | 0.04 |\n| AGN_LIKE | 3334650 | AT2025ssi | 99.146548 | -37.499065 | 20.09 | 0.05 | N/A | N/A | 19.68 | 0.03 | N/A | N/A |\n\nAdditionally, we also recover two of the candidates reported by SWIFT-XRT (GCN 41201). One is the known blazar BL Lacertae (S250727dc_X1), which has been mentioned to be highly variable. The other (S250727dc_X7) is the galaxy ESO 365-5. Both of these galaxies have been spectroscopically measured to have redshifts z=0.055 (García-Pérez et al. 2024) and z=0.045 (Jones et al. 2009) respectively, which both lie outside of the LVK localization volume.\n\n| TYPE | ID | ATNAME | RA | DEC | MAG_G | MAG_G_ERR | MAG_R | MAG_R_ERR | MAG_I | MAG_I_ERR | MAG_Z | MAG_Z_ERR |\n|----------|---------|--------|------------|-----------|-------|-----------|-------|-----------|-------|-----------|-------|-----------|\n| AGN_LIKE | 3334338 | S250727dc_X1 | 96.778098 | -35.487631 | 17.32 | 0.01 | 16.98 | 0.01 | N/A | N/A | 16.5 | 0.0 |\n| AGN_LIKE | 3334206 | S250727dc_X7 | 94.688512 | -35.304098 | 18.79 | 0.02 | 18.32 | 0.02 | 18.34 | 0.01 | 18.51 | 0.02 |\n\nWe do not recover any of the other candidates reported by SWIFT-XRT, nor the transient AT2025smm reported by SWIFT-UVOT (GCN 41187).\n\nThe DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide-ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.",
"published": "2025-07-31T13:19:19.605601Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-31T13:19:19.605637Z",
"modified": "2025-07-31T13:19:19.618290Z",
"targets": []
},
{
"id": 2240388,
"topic": "gcn.circulars",
"uuid": "a95c4bf7-aba7-4832-9d04-f14e8a04293a",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41205"
},
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT follow up of AT 2025smm",
"createdOn": 1753907305327,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41205,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DTU Space), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift re-observed the location of AT 2025smm in the high-probability region\nof LIGO/Virgo/KAGRA S250725dc from 1.56 to 1.63 d after the trigger. The\nsource is no longer detected. we obtained the following 3-sigma upper\nlimits:\n\nFilter Exposure(s) 3-sigma UL AB mag)\nUVW2 606 22.3\nUVM2 466 21.8\nUVW1 303 21.5\n U 151 20.8\n B 151 20.1\n V 151 19.3\n\nNo corrections were made for interstellar extinction.\n",
"published": "2025-07-30T20:28:32.955329Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-30T20:28:32.955344Z",
"modified": "2025-07-30T20:28:32.961760Z",
"targets": []
},
{
"id": 2239496,
"topic": "gcn.circulars",
"uuid": "51b406a0-e555-47fe-a197-cf724303a09b",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"authors": "Paul Kuin at MSSL <npkuin@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41187"
},
"format": "text/plain",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-UVOT detection of a new source, AT 2025smm",
"version": 2,
"editedBy": "Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Paul Kuin at MSSL <npkuin@gmail.com>",
"editedOn": 1753906880999,
"createdOn": 1753716685802,
"submitter": "Paul Kuin at MSSL <npkuin@gmail.com>",
"circularId": 41187,
"submittedHow": "email"
},
"message_text": "N.P.M. Kuin (UCL-MSSL), M.H. Siegel (PSU), A.A. Breeveld (UCL-MSSL), C.\nGronwall (PSU), N.J. Klingler (NASA/GSFC UMBC CRESST II),F.E. Marshall\n(NASA/GSFC), S.R. Oates (Lancaster U.), M. De Pasquale (University of\nMessina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), A. D’Aì\n(INAF-IAFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G.\nBernardini (INAF-OAB),S. Campana (INAF-OAB), S.B. Cenko (NASA/GSFC), J.J.\nDelaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR),\nR.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri\n(PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha\n(NASA/GSFC),H.A. Krimm (NSF), D. B. Malesani (DTU Space), P. O’Brien (U.\nLeicester), J.P. Osborne (U. Leicester), T. Parsotan (NASA/GSFC), M. Perri\n(ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T.\nSbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB),\nE. Troja (U. Tor Vergata, INAF), and A. Tohuvavohu (U. Toronto)report:\n\nSwift observed the high-probability region of LIGO/Virgo/KAGRA S250725dc\nstarting 65.3 minutes after the trigger date. A new UVOT source was found\nin the U band with U = 19.9 +/- 0.3 mag (ABmag) image taken on 2025-07-28\n00:34_55 UT at the following position:\n\nRA = 97.47619, Dec = -37.23307 deg, or RA = 06:29:54.3,\nDec = -37:13:55.45 in sexagesimal notation.\n\nWe do not find any source on that location in DSS, using the Vizier\nphotometry tool, or as a nearby object known to the Minor Planet Center\nMPChecker tool. The object has been registered as 2025smm by the Transient\nName Server.\n\nA further Swift observation has been planned.\n",
"published": "2025-07-30T20:21:21.965553Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-30T20:21:21.965570Z",
"modified": "2025-07-30T20:21:21.971096Z",
"targets": []
},
{
"id": 2214900,
"topic": "gcn.circulars",
"uuid": "8cb9b651-3f5f-449b-9540-9f8e32db5e31",
"title": "LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO",
"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/41202"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Upper limits from Swift/BAT-GUANO",
"createdOn": 1753897772173,
"submitter": "Maia Williams at PSU <mjw6837@psu.edu>",
"circularId": 41202,
"submittedHow": "web"
},
"message_text": "Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman, James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:\n\nSwift/BAT was observing 100.00% of the GW localization probability ([Bilby.offline0.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250727dc/files/Bilby.offline0.multiorder.fits)) at merger time. A fraction 100.00% of the GW localization posterior is contained inside the BAT coded FoV.\n\nThe LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [Tohuvavohu et al. 2020, ApJ, 900, 1](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).\n\nUpon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.\n\nUsing the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)) we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.\n\nWe quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.\nIn units of 10^-7 erg/s/cm^2:\n\n|time_bin (s) |soft |normal|hard |GRB170817\n|-|-|-|-|-|\n|0.256 |0.56 |0.87 |0.79 |0.92 \n|1.024 |0.28 |0.44 |0.40 |0.47 \n|4.096 |0.15 |0.24 |0.22 |0.25 \n|16.384 |0.10 |0.15 |0.14 |0.16 \n\n\nThe upper limits as function of sky position are plotted here, alongside the GW localization: https://zenodo.org/records/16612515\n\nThe solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.\n\nGUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.\n\nA live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/",
"published": "2025-07-30T17:49:41.160946Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-30T17:49:41.160961Z",
"modified": "2025-07-30T17:49:41.170499Z",
"targets": []
},
{
"id": 2182066,
"topic": "gcn.circulars",
"uuid": "b6dcb855-ac7d-47e4-afa5-da02c2ce7871",
"title": "LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources",
"submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"authors": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41201"
},
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: Swift-XRT observations, 8 X-ray sources",
"createdOn": 1753872329291,
"submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
"circularId": 41201,
"submittedHow": "email"
},
"message_text": "P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko\n(NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P.\nBeardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S.\nCampana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina), S.\nDichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V. D’Elia\n(ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N.\nKlingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha\n(NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O’Brien\n(U. Leicester), M.J. Page (UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato\n(INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),\nE. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:\n\nSwift has carried out 171 observations of the LVK error region for the GW trigger\nS250727dc convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),\nusing the 'bayestar.multiorder.fits,1' GW localisation map. As this is a 3D skymap,\ngalaxy distances were taken into account in selecting which ones to observe. The\nobservations currently span from 3.9 ks to 157 ks after the LVK trigger, and the XRT\nhas covered 11.5 deg^2 on the sky (corrected for overlaps). This covers 70% of the\nprobability in the imrpoved 'Bilby.offline0.multiorder.fits' skymap, and 69% after\nconvolving with the 2MPZ galaxy catalogue, as described by Evans et al. (2016,\nMNRAS,\t462, 1591).\n\nIn total, we have detected 8 X-ray sources. Each source is assigned a rank of 1-4\nwhich describes how likely it is to be related to the GW trigger, with 1 being the\nmost likely and 4 being the least likely. The ranks are described at\nhttps://www.swift.ac.uk/ranks.php.\n\nWe have found:\n\n * 0 sources of rank 1\n * 0 sources of rank 2 (see note)\n * 3 sources of rank 3\n * 5 sources of rank 4\n\nNOTE: The online system reports a rank 2 source: Source S250727dc_X1. However, this\nin the known blazar BL Lac (the prototype of its class of object). During our\nobservation, its flux was significantly increased compared to the mean value in the\nLSXPS catalogue, hence the automated classification. However, it is known to be\nhighly variable (e.g.\nhttps://www.swift.psu.edu/monitoring/source.php?source=BLLacertae) and the\nintensities in LSXPS are drawn solely from PC-mode data so are biased towards the\nlower-intensity states. We have thus reclassified this as rank 4 for this circular.\n\n\nRANK 3 sources\n==============\n\nThese are uncatalogued X-ray sources, however they are not brighter than previous\nupper limits, so do not stand out as likely counterparts to the GW trigger.\n\n| Source ID\t | RA\t\t | Dec \t | Err90 | Detection Flag\t|\n| S250727dc_X5 | 06h 23m 38.04s | -35d 38' 24.1\" |\t7.7\" |\t\t GOOD |\n| S250727dc_X6 | 06h 28m 07.71s | -37d 10' 10.3\" |\t9.5\" |\t REASONABLE |\n| S250727dc_X7 | 06h 18m 45.03s | -35d 18' 15.1\" |\t7.0\" |\t\t GOOD |\n\n\nRANK 4 sources\n==============\n\nThese are catalogued X-ray sources, showing no signs of outburst compared to\nprevious observations, so they are not likely to be related to the GW trigger.\n\n| Source ID\t | RA\t\t | Dec \t | Err90 | Detection Flag\t|\n| S250727dc_X1 | 06h 27m 06.55s | -35d 29' 15.0\" |\t3.8\" |\t\t GOOD |\n| S250727dc_X3 | 06h 34m 35.33s | -37d 17' 12.1\" |\t8.3\" |\t\t GOOD |\n| S250727dc_X8 | 06h 36m 18.37s | -37d 22' 33.4\" |\t4.5\" |\t\t GOOD |\n| S250727dc_X9 | 06h 19m 53.43s | -35d 24' 28.2\" |\t7.7\" |\t\t GOOD |\n| S250727dc_X10 | 06h 20m 08.64s | -34d 37' 54.4\" |\t5.8\" |\t\t GOOD |\n\n\nIn addition, we took a 1.9 ks exposure of the UVOT source reported by Kuin et al.\n(GCN Circ. 41187). No XRT counterpart was found, with a 3-sigma upper limit of\n3.9e-3 ct/sec (approx 1.6e-13 erg cm^-s s^-1, 0.3-10 keV).\n\nThe results of the XRT automated analysis, including details of the sources listed\nabove, are online at https://www.swift.ac.uk/LVC/S250727dc\n\nThis circular is an official product of the Swift XRT team.\n\n\n",
"published": "2025-07-30T10:45:37.102708Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-30T10:45:37.102733Z",
"modified": "2025-07-30T10:45:37.114370Z",
"targets": []
},
{
"id": 2166848,
"topic": "gcn.circulars",
"uuid": "5551c2aa-9b1f-450a-b1ee-8a26df30ad25",
"title": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm",
"submitter": "xjh@andrew.cmu.edu",
"authors": "xjh@andrew.cmu.edu",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41197"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: DECam GW-MMADS upper limits of AT 2025smm",
"createdOn": 1753822532121,
"submitter": "xjh@andrew.cmu.edu",
"circularId": 41197,
"submittedHow": "web"
},
"message_text": "Xander J. Hall (CMU), Lei Hu (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Keerthi Kunnumkai (CMU), Brendan O’Connor (CMU), on behalf of the Gravitational Wave MultiMessenger Astronomy DECam Survey (GW-MMADS) team\n\nDECam observed the southern high probability area of the LVK gravitational wave candidate S250727dc (GCN 41179) using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope, as part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS; PI: Andreoni & Palmese). Observations started at 2025-07-29 09:39 UTC and cover the 90% probability area (GCN 41188).\n\nWe inspected the location of AT 2025smm, reported by Swift UVOT (GCN 41187), we find no sources in the ugri-bands. Here we report the following limiting magnitudes, not corrected for Galactic extinction:\n\n| Filter | Mag (AB) |\n|--------|----------|\n| g | > 22.15 |\n| r | > 21.46 | \n| i | > 20.60 | \n\nFurther analysis is underway.\n\nWe thank the CTIO and NOIRLab staff for supporting these observations and the data calibration.\n",
"published": "2025-07-29T20:55:40.191435Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-29T20:55:40.191451Z",
"modified": "2025-07-29T20:55:40.200463Z",
"targets": []
},
{
"id": 2157125,
"topic": "gcn.circulars",
"uuid": "6aee230b-e872-431e-a740-0224242bad36",
"title": "LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm",
"submitter": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"authors": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"data": {
"urls": {
"gcn_circular": "https://gcn.nasa.gov/circulars/41196"
},
"format": "text/markdown",
"$schema": "https://gcn.nasa.gov/schema/v4.4.0/gcn/circulars.schema.json",
"eventId": "LIGO/Virgo/KAGRA S250727dc",
"subject": "LIGO/Virgo/KAGRA S250727dc: PRIME J-band upper limits of AT 2025smm",
"createdOn": 1753812667976,
"submitter": "Joe Durbak at UMD <gcn.joedurbak@gmail.com>",
"circularId": 41196,
"submittedHow": "web"
},
"message_text": "LIGO/Virgo/KAGRA S250727dc : PRIME J-band upper limits of AT 2025smm\n\nJ. Durbak (UMD), O. Guiffreda (UMD), N. Passaleva, M. El Kabir, A. S. Kutyrev (NASA/GSFC), E. Troja (U Rome), S. B. Cenko (NASA/GSFC)\n\nFollowing the LIGO/Virgo/KAGRA detection of S250727dc (GCN 41179), we observed a portion of the transient field in J-filter with PRIME ~32 hours after LVK detection. This field was chosen to overlap with the Swift UVOT detection of an uncatalogued source, AT 2025smm (GCN 41187).\n\nAt the position of AT 2025smm reported by Swift UVOT (GCN 41187), we detect no sources in J-band. Using nearby 2MASS stars for preliminary calibration we derive the following limiting magnitude, not corrected for Galactic extinction: \n\n| Filter | Mag(AB) |\n|--------|--------------|\n| J | > 19.7 | \n\nPRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).\n\nWe thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.",
"published": "2025-07-29T18:11:16.987199Z",
"message_parser": "GCN Circular Parser v2",
"retracted": false,
"created": "2025-07-29T18:11:16.987218Z",
"modified": "2025-07-29T18:11:16.995854Z",
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}
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"title": "S250830cb - PRELIMINARY",
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{
"id": 5776680,
"topic": "gcn.circulars",
"uuid": "5c179e7e-ba4f-4efd-b842-d6ab54e8f5c0",
"title": "LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate",
"submitter": "Jyotirmaya Mohanta at University of Tsukuba <s2430161@u.tsukuba.ac.jp>",
"authors": "Jyotirmaya Mohanta at University of Tsukuba <s2430161@u.tsukuba.ac.jp>",
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"eventId": "LIGO/Virgo/KAGRA S250827l",
"subject": "LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate",
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"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe identified the compact binary merger candidate S250827l during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 01:27:57.538 UTC (GPS time: 1440293295.538). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.\n\nS250827l is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:\n\nhttps://gracedb.ligo.org/superevents/S250827l\n\nThe classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).\n\nAssuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.\n\nThe source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.\n\nTwo sky maps are available at this time and can be retrieved from the GraceDB event page:\n * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time.\n * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.\n\nThe preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1145 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2132 +/- 580 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004\n [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018\n [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008\n [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234\n [5] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034\n [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a\n [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023\n [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe\n [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013\n",
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{
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"title": "LIGO/Virgo/KAGRA S250827l: Identification of a GW compact binary merger candidate",
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"message_text": "The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:\n\nWe identified the compact binary merger candidate S250827l during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-27 01:27:57.538 UTC (GPS time: 1440293295.538). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], MLy [5], PyCBC Live [6], and SPIIR [7] analysis pipelines.\n\nS250827l is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:\n\nhttps://gracedb.ligo.org/superevents/S250827l\n\nThe classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).\n\nAssuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is <1%. [8] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [8] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.\n\nThe source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.\n\nTwo sky maps are available at this time and can be retrieved from the GraceDB event page:\n * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 32 seconds after the candidate event time.\n * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [9], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.\n\nThe preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 1145 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2132 +/- 580 Mpc (a posteriori mean +/- standard deviation).\n\nFor further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.\n\n [1] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004\n [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018\n [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008\n [4] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234\n [5] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034\n [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a\n [7] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023\n [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe\n [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013\n",
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