LIGO/Virgo/KAGRA S240917cb: Identification of a GW compact binary merger candidate

We identified the compact binary merger candidate S240917cb during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2024-09-17 13:02:37.724 UTC (GPS time: 1410613375.724). The candidate was found by the GstLAL [1] and MBTA [2] analysis pipelines. S240917cb is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.4e-08 Hz, or about one in 7 months. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S240917cb 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 for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 6%. Three sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 2 hours after the candidate event time. * bayestar.multiorder.fits,3, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 2 hours after the candidate event time. * bayestar.multiorder.fits,4, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 2 hours after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,4. For the bayestar.multiorder.fits,4 sky map, the 90% credible region is 3663 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 8510 +/- 2727 Mpc (a posteriori mean +/- standard deviation). The first preliminary notice was delayed by a technical issue that also caused the second preliminary notice to erroneously include Virgo data in the alert. The distributed skymap bayestar.multiorder.fits,3 included Virgo data and should be disregarded. 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/. [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625 [2] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013