TITLE: GCN CIRCULAR NUMBER: 34857 SUBJECT: LIGO/Virgo/KAGRA S231020bw: Identification of a GW compact binary merger candidate DATE: 23/10/20 19:25:05 GMT FROM: Amanda Baylor at University of Wisconsin-Milwaukee <amanda.baylor@ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S231020bw during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-10-20 18:05:09.780 UTC (GPS time: 1381860327.780). The candidate was found by the GstLAL [1] analysis pipeline. S231020bw is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.5e-10 Hz, or about one in 91 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S231020bw The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (<1%). The data is consistent with either a known class of instrumental artifacts in one detector or a GW signal that is located in the sky at the null point of the other detector. The skymap for this candidate is therefore the 2 null points of the second detector, where the detector is the least sensitive to gravitational waves. 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 26 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 888 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2868 +/- 924 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/34857. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...