LIGO/Virgo/KAGRA S230529ay: Identification of a GW compact binary merger candidate

We identified the compact binary merger candidate S230529ay during real-time processing of data from LIGO Livingston Observatory (L1) at 2023-05-29 18:15:00.746 UTC (GPS time: 1369419318.746). The candidate was found by the GstLAL [1] and PyCBC Live [2] analysis pipelines. S230529ay is an event of interest because its false alarm rate, as estimated by the online analysis, is 2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S230529ay The classification of the GW signal, in order of descending probability is NSBH (62%), BNS (31%), Terrestrial (7%), or BBH (<1%). Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 12%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that any one of the binary components lie between 3 to 5 solar mass (HasMassgap) is 98%. 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 25 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 well fit by an ellipse with an area of 31171 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis): icrs; ellipse(00h00m, +00d00m, 163.70d, 97.20d, 147.56d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 217 +/- 71 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. arXiv:2305.06286 (2023) and Ewing et al. arXiv:2305.05625 (2023) [2] Dal Canton et al. ApJ 923, 254 (2021) [3] Chatterjee et al. The Astrophysical Journal 896, 1 (2020) [4] Singer & Price PRD 93, 024013 (2016)