GCN Circular 36340

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S240428dr during real-time processing of data from LIGO Hanford Observatory (H1) and Virgo Observatory (V1) at 2024-04-28 22:54:40.077 UTC (GPS time: 1398380098.077). The candidate was found by the GstLAL [1] analysis pipeline.
S240428dr is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.1e-14 Hz, or about one in 1e6 years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S240428dr
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%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] 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%.
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 [3], distributed via GCN notice about 29 seconds after the candidate event time.
bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], 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 286 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 765 +/- 177 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/.
 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004
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 and Ewing et al. (2023) arXiv:2305.05625
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 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
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 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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