TITLE: GCN CIRCULAR NUMBER: 35445 SUBJECT: LIGO/Virgo/KAGRA S231231ag: Identification of a GW compact binary merger candidate DATE: 23/12/31 16:10:22 GMT FROM: Keita Kawabe at LIGO Hanford <kkawabe@caltech.edu> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S231231ag during real-time processing of data from LIGO Hanford Observatory (H1) at 2023-12-31 15:40:16.670 UTC (GPS time: 1388072434.670). The candidate was found by the GstLAL [1] analysis pipeline. S231231ag is an event of interest because its false alarm rate, as estimated by the online analysis, is 8.4e-15 Hz, or about one in 1e7 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S231231ag 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 the lighter compact object is consistent with a neutron star 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] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassgap) is <1%. 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 [3], distributed via GCN notice about 27 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 5 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 24218 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1114 +/- 357 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/userguide/. [1] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625 (2023) [2] Chatterjee et al. ApJ 896, 54 (2020) [3] Singer & Price PRD 93, 024013 (2016) View this GCN Circular online at https://gcn.nasa.gov/circulars/35445. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...