LIGO/Virgo/KAGRA S231113bb: Identification of a GW compact binary merger candidate

We identified the compact binary merger candidate S231113bb during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-13 12:26:23.884 UTC (GPS time: 1383913601.884). The candidate was found by the GstLAL [1] and MBTA [2] analysis pipelines. S231113bb is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.6e-08 Hz, or about one in 6 months. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S231113bb The classification of the GW signal, in order of descending probability, is BBH (96%), Terrestrial (4%), 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 <1%. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 27 seconds after the candidate event time. * bayestar.multiorder.fits,2, 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,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 2716 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4900 +/- 1642 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] Aubin et al. CQG 38, 095004 (2021) [3] Chatterjee et al. ApJ 896, 54 (2020) [4] Singer & Price PRD 93, 024013 (2016)