TITLE: GCN CIRCULAR NUMBER: 34293 SUBJECT: LIGO/Virgo/KAGRA S230729z: Identification of a GW compact binary merger candidate DATE: 23/07/29 11:17:18 GMT FROM: Angélique Lartaux at IJCLab The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S230729z during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-07-29 08:23:17.201 UTC (GPS time: 1374654215.201). The candidate was found by the GstLAL [1], MBTA [2], and PyCBC Live [3] analysis pipelines. S230729z is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.4e-09 Hz, or about one in 9 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S230729z The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), 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%. [4] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [4] 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 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 [5], distributed via GCN notice about 35 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], 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 1428 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1546 +/- 472 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] Aubin et al. CQG 38, 095004 (2021) [3] Dal Canton et al. ApJ 923, 254 (2021) [4] Chatterjee et al. ApJ 896, 54 (2020) [5] Singer & Price PRD 93, 024013 (2016)