TITLE: GCN CIRCULAR NUMBER: 34520 SUBJECT: LIGO/Virgo/KAGRA S230822bm: Identification of a GW compact binary merger candidate DATE: 23/08/22 23:50:00 GMT FROM: Hideyuki Tagoshi at ICRR/U of Tokyo/KAGRA The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S230822bm during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-08-22 23:03:37.497 UTC (GPS time: 1376780635.497). The candidate was found by the CWB [1] and GstLAL [2] analysis pipelines. S230822bm is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.6e-08 Hz, or about one in 1 year, 2 months. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S230822bm The classification of the GW signal, in order of descending probability, is BBH (98%), Terrestrial (2%), 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%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [3] 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 4%. 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 [4], distributed via GCN notice about 27 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], 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 4472 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6854 +/- 2325 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] Klimenko et al. PRD 93, 042004 (2016) [2] Tsukada et al. arXiv:2305.06286 (2023) and Ewing et al. arXiv:2305.05625 (2023) [3] Chatterjee et al. ApJ 896, 54 (2020) [4] Singer & Price PRD 93, 024013 (2016)