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[vsnet-grb-info 40696] LIGO/Virgo/KAGRA S251108fi: DDOTI Optical Candidate
by GCN Circulars 10 Nov '25

10 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42632 SUBJECT: LIGO/Virgo/KAGRA S251108fi: DDOTI Optical Candidate DATE: 25/11/10 07:18:02 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra(a)astro.unam.mx> Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report: We observe the field of the gravitational wave candidate S251108fi (LVKC, GCN Circ. 42624) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on the Sierra of San Pedro Martir (http://ddoti.astroscu.unam.mx) on the night of 2025-11-10 UTC. DDOTI observed from 02:22 UTC to 04:35 UTC (T+31.4 h to T+33.6 h after the trigger), with a total exposure time of 72 minutes, pointing the region reported by the IceCube Collaboration (GCN Circ. 42630). Comparing our observations with the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we identify an uncatalogued source at: RA = 00:57:11.36 (14.2973 d) DEC= +07:39:07.1 (7.6520 d) with a preliminary magnitude AB magnitude of: w = 20.11 +/- 0.11. This source lies outside the IceCube region but within the 90% credible region reported by the LVK Collaboration (GCN Circ. 42628). It is located approximately 4" from a known galaxy with a photometric redshift of z = 0.214 ± 0.031 in the Legacy Survey DR10 (Dey et al. 2019). If they are associated, such a redshift would make a connection with the GW candidate unlikely. However, further observations and analysis are ongoing. This value is not corrected for the Galactic extinction. We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/42632. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40695] GRB 251105A: AstroSat CZTI detection
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42631 SUBJECT: GRB 251105A: AstroSat CZTI detection DATE: 25/11/10 05:40:22 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> A. Arya (IITB), A. Goyal (IITB), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 251105A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 42596), and Swift BAT (J. DeLaunay et al. 2025, GCN 42593). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-11-05 04:41:19.50 UTC. The measured peak count rate associated with the burst is 85 (+32, -10) counts/s above the background in the combined data of three quadrants (out of four), with a total of 1064 (+275, -254) counts. The local mean background count rate was 211 (+3, -3) counts/s. Using cumulative rates, we measure a T90 of 28 (+5, -5) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-11-05 04:41:20.32 UTC. The measured peak count rate associated with the burst is 178 (+72, -15) counts/s above the background in the combined data of all quadrants, with a total of 2223 (+423, -516) counts. The local mean background count rate was 1203 (+6, -5) counts/s. We measure a T90 of 28 (+2, -7) s from the cumulative Veto light curve. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/42631. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40694] LIGO/Virgo/KAGRA S251108fi: One counterpart neutrino candidate from IceCube neutrino searches
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42630 SUBJECT: LIGO/Virgo/KAGRA S251108fi: One counterpart neutrino candidate from IceCube neutrino searches DATE: 25/11/09 22:42:06 GMT FROM: Alicia Mand at IceCube/UW-Madison <aemand(a)wisc.edu> IceCube Collaboration (http://icecube.wisc.edu/) reports: Searches for track-like muon neutrino events detected by IceCube consistent with the sky localization of gravitational-wave candidate S251108fi (GCN 42628) in a time range of 1000 seconds centered on the alert event time (2025-11-08 19:05:08.506 UTC to 2025-11-08 19:21:48.506 UTC) have been performed [1,2]. During this time period IceCube was collecting good quality data. Two hypothesis tests were conducted. The first search is a maximum likelihood analysis which searches for a generic point-like neutrino source coincident with the given GW skymap. The second uses a Bayesian approach to quantify the joint GW + neutrino event significance, which assumes a binary merger scenario and accounts for known astrophysical priors, such as GW source distance, in the significance estimate [3]. One track-like event is found in spatial and temporal coincidence with the gravitational-wave candidate S251108fi calculated from the map circulated in the 4-Update notice. This represents an overall p-value of 0.0076 from the generic transient search and an overall p-value of 0.0421 for the Bayesian search. These p-values measure the consistency of the observed track-like events with the known atmospheric backgrounds for this single map (not trials corrected for multiple GW events). The most probable multi-messenger source direction based on the neutrinos and GW skymap is RA 15.12, Dec 9.06 degrees. The reported p-values can differ due to the estimated distance of the GW candidate. The distance is used as a prior in the Bayesian binary merger search, while it is not taken into account in the generic transient point-like source search. The false alarm rate of these coincidences can be obtained by multiplying the p-values with their corresponding GW trigger rates. Further details are available at https://gcn.nasa.gov/missions/icecube. Additional details and updates will be posted at https://roc.icecube.wisc.edu/public/LvkNuTrackSearch/. Properties of the coincident event are shown below. | dt(s) | RA(deg) | Dec(deg) | Angular uncertainty(deg)| p-value(generic transient)| p-value(Bayesian) | |-------|----------|----------|-------------------------|---------------------------|-----------------------| |-242.48 | 15.01 | 9.06 | 1.13 | 0.0076 | 0.0421 | where: dt = Time of track event minus time of GW trigger (sec). Angular uncertainty = Angular uncertainty of track event: the radius of a circle representing 90% CL containment by area. p-value = the p-value for this specific track event from each search. Event p-values are provided when the per-event p-value is less than 0.1 in either search. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu [1] M. G. Aartsen et al 2020 ApJL 898 L10 [2] Abbasi et al. Astrophys.J. 944 (2023) 1, 80 [3] I. Bartos et al. 2019 Phys. Rev. D 100, 083017 View this GCN Circular online at https://gcn.nasa.gov/circulars/42630. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40693] IceCube-Cascade 251107A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42629 SUBJECT: IceCube-Cascade 251107A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/11/09 19:33:46 GMT FROM: Yuhua Yao at IceCube/UW-Madison <yyao255(a)icecube.wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-Cascade 251107A (https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/141568_42562719.amon) in a time range of 1000 seconds centered on the alert event time (2025-11-07 20:53:25.880 UTC to 2025-11-07 21:10:05.880 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-Cascade 251107A. We report a p-value of 1.00 in this time window. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, ranges from 1.2e+01 to 2.5e+01 GeV cm^-2 within the 90% spatial containment region of IceCube-Cascade 251107A in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 4e+04 GeV and 4e+06 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-11-06 21:01:45.880 UTC to 2025-11-08 21:01:45.880 UTC). In this case, we report a p-value of 0.57, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, ranges from 1.2e+01 to 2.7e+01 GeV cm^-2 within the 90% spatial containment region of IceCube-Cascade 251107A in a 2 day time window. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu. [1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/42629. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40692] IceCube-Cascade 251107A: Upper limits from a search for additional neutrino events in IceCube
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42629 SUBJECT: IceCube-Cascade 251107A: Upper limits from a search for additional neutrino events in IceCube DATE: 25/11/09 19:33:46 GMT FROM: Yuhua Yao at IceCube/UW-Madison <yyao255(a)icecube.wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-Cascade 251107A (https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/141568_42562719.amon) in a time range of 1000 seconds centered on the alert event time (2025-11-07 20:53:25.880 UTC to 2025-11-07 21:10:05.880 UTC) during which IceCube was collecting good quality data. Excluding the event that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-Cascade 251107A. We report a p-value of 1.00 in this time window. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, ranges from 1.2e+01 to 2.5e+01 GeV cm^-2 within the 90% spatial containment region of IceCube-Cascade 251107A in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 4e+04 GeV and 4e+06 GeV. A subsequent search was performed including 2 days of data centered on the alert event time (2025-11-06 21:01:45.880 UTC to 2025-11-08 21:01:45.880 UTC). In this case, we report a p-value of 0.57, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, ranges from 1.2e+01 to 2.7e+01 GeV cm^-2 within the 90% spatial containment region of IceCube-Cascade 251107A in a 2 day time window. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu. [1] IceCube Collaboration, R. Abbasi et al., ApJ 910 4 (2021) View this GCN Circular online at https://gcn.nasa.gov/circulars/42629. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40691] LIGO/Virgo/KAGRA S251108fi: Updated Sky localization
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42628 SUBJECT: LIGO/Virgo/KAGRA S251108fi: Updated Sky localization DATE: 25/11/09 08:53:26 GMT FROM: Abhishek Sharma at IIT Gandhinagar <abhishek.sharma(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We 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 S251108fi (GCN Circular 42624). 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: https://gracedb.ligo.org/superevents/S251108fi For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 528 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2972 +/- 957 Mpc (a posteriori mean +/- standard deviation). For 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/. [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 View this GCN Circular online at https://gcn.nasa.gov/circulars/42628. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40690] LIGO/Virgo/KAGRA S251108dn: Updated Sky localization
by GCN Circulars 09 Nov '25

09 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42627 SUBJECT: LIGO/Virgo/KAGRA S251108dn: Updated Sky localization DATE: 25/11/09 08:48:37 GMT FROM: Abhishek Sharma at IIT Gandhinagar <abhishek.sharma(a)ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We 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 S251108dn (GCN Circular 42619). 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: https://gracedb.ligo.org/superevents/S251108dn For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 370 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1089 +/- 246 Mpc (a posteriori mean +/- standard deviation). For 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/. [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 View this GCN Circular online at https://gcn.nasa.gov/circulars/42627. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40689] GRB 251108A: Fermi GBM Final Localization Correction
by GCN Circulars 08 Nov '25

08 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42626 SUBJECT: GRB 251108A: Fermi GBM Final Localization Correction DATE: 25/11/09 00:49:37 GMT FROM: Bagrat Mailyan at Florida Tech <mbagrat(a)gmail.com> The Fermi GBM team reports the detection of a likely SHORT GRB "At 21:19:46.26 UT on 08 November 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251108A (trigger 784329591/251108889). This trigger was initially classified as GRB by the flight software. The on-ground calculated location, using the Fermi GBM trigger data, is RA = 144.63, Dec = 8.03 (J2000 degrees, equivalent to J2000 9h 39m, +8d 2'), with a statistical uncertainty of 3.23 degrees. The angle from the Fermi LAT boresight is 10 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251108889/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251108889/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251108889/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/42626. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40688] Fermi trigger No 784329591: Global MASTER-Net observations report
by GCN Circulars 08 Nov '25

08 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42625 SUBJECT: Fermi trigger No 784329591: Global MASTER-Net observations report DATE: 25/11/08 22:16:41 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB251108.89 (trigger No 784329591,09h 39m 43.20s , +04d 40m 48.0s, R=4.06) errorbox 1896 sec after notice time and 1931 sec after trigger time at 2025-11-08 21:51:57 UT, with upper limit up to 16.0 mag. The observations began at zenith distance = 79 deg. The sun altitude is -61.0 deg. The galactic latitude b = 40 deg., longitude l = 231 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3036088 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 1961 | 2025-11-08 21:51:57 | MASTER-Kislovodsk | (09h 27m 10.24s , +08d 52m 53.0s) | C | 60 | 15.4 | 1961 | 2025-11-08 21:51:57 | MASTER-Kislovodsk | (09h 31m 07.31s , +08d 30m 22.5s) | C | 60 | 15.5 | 2041 | 2025-11-08 21:53:17 | MASTER-Kislovodsk | (09h 34m 32.46s , +10d 24m 18.7s) | C | 60 | 16.0 | 2120 | 2025-11-08 21:54:36 | MASTER-Kislovodsk | (09h 24m 35.66s , +07d 00m 49.3s) | C | 60 | 15.4 | 2120 | 2025-11-08 21:54:36 | MASTER-Kislovodsk | (09h 28m 31.61s , +06d 38m 17.4s) | C | 60 | 15.6 | 2361 | 2025-11-08 21:58:36 | MASTER-Kislovodsk | (09h 22m 30.89s , +05d 06m 23.8s) | C | 60 | 15.2 | 2361 | 2025-11-08 21:58:37 | MASTER-Kislovodsk | (09h 26m 26.13s , +04d 43m 50.1s) | C | 60 | 15.4 | 2829 | 2025-11-08 22:06:25 | MASTER-Kislovodsk | (09h 42m 37.63s , +08d 54m 28.6s) | C | 60 | 15.7 | 2829 | 2025-11-08 22:06:25 | MASTER-Kislovodsk | (09h 46m 36.13s , +08d 31m 13.0s) | C | 60 | 15.9 | 2909 | 2025-11-08 22:07:44 | MASTER-Kislovodsk | (09h 21m 08.44s , +03d 14m 02.3s) | C | 60 | 14.8 | 2909 | 2025-11-08 22:07:44 | MASTER-Kislovodsk | (09h 25m 04.69s , +02d 50m 48.6s) | C | 60 | 14.8 | 3148 | 2025-11-08 22:11:43 | MASTER-Kislovodsk | (09h 39m 56.05s , +07d 00m 48.8s) | C | 60 | 15.5 | 3148 | 2025-11-08 22:11:44 | MASTER-Kislovodsk | (09h 43m 53.52s , +06d 37m 32.3s) | C | 60 | 15.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/42625. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 40687] LIGO/Virgo/KAGRA S251108fi: Identification of a GW compact binary merger candidate
by GCN Circulars 08 Nov '25

08 Nov '25

TITLE: GCN CIRCULAR NUMBER: 42624 SUBJECT: LIGO/Virgo/KAGRA S251108fi: Identification of a GW compact binary merger candidate DATE: 25/11/08 19:45:25 GMT FROM: joshua.freed(a)ligo.org The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S251108fi during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-11-08 19:13:28.506 UTC (GPS time: 1446664426.506). The candidate was found by the Aframe [1], cWB [2], cWB BBH [3], GstLAL [4], MBTA [5], and PyCBC Live [6] analysis pipelines. S251108fi is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-12 Hz, or about one in 1e4 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S251108fi The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%). Assuming 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%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] 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 source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 25 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN and SCiMMA notices about 4 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1524 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3537 +/- 1032 Mpc (a posteriori mean +/- standard deviation). For 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/. [1] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010 [2] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [3] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [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 [5] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [6] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/42624. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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