GCN Circular 33838

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S230520ae during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-05-20 22:48:42.560 UTC (GPS time: 1368658140.560). The candidate was found by the MBTA [1], CWB [2], PyCBC Live [3], SPIIR [4], and GstLAL [5] analysis pipelines. S230520ae is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.1e-09 Hz, or about one in 10 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S230520ae The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%). Assuming the candidate is astrophysical in origin, the probability that the lighter compact object has a mass < 3 solar masses (HasNS) is <1%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] 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 [7], distributed via GCN notice about 29 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], 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 1716 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1974 +/- 583 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] Adams et al. CQG 33, 175012 (2016) [2] Klimenko et al. PRD 93, 042004 (2016) [3] Dal Canton et al. ApJ 923, 254 (2021) [4] Qi Chu, PhD Thesis, The University of Western Australia (2017) [5] Messick et al. PRD 95, 042001 (2017) [6] Chatterjee et al. The Astrophysical Journal 896, 1 (2020) [7] Singer & Price PRD 93, 024013 (2016)