LIGO/Virgo/KAGRA S240426dl: Identification of a GW compact binary merger candidate

We identified the compact binary merger candidate S240426dl during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2024-04-26 15:18:03.773 UTC (GPS time:1398179901.773). The candidate was found by the CWB[1],GstLAL [2], and MBTA [3] analysis pipelines. S240426dl is an event of interest because its false alarm rate, as estimated by the online analysis, is 5.2e-08 Hz, or about one in 7 months. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S240426dl The classification of the GW signal, in order of descending probability, is Terrestrial (70%), BBH (30%), BNS (<1%), or NSBH (<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 <1%. Three 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 25 seconds after the candidate event time. * cwb.multiorder.fits,0, an initial localization generated by cWB [1], distributed via GCN notice about 2 minutes after the candidate event time. * cwb.multiorder.fits,1, an initial localization generated by cWB [1], distributed via GCN notice about 7 minutes after the candidate event time. The preferred sky map at this time is cwb.multiorder.fits,1. For the cwb.multiorder.fits,1 sky map, the 90% credible region is 2024 deg2. 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625 [3] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913 [4] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [5] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013