SUBJECT: LIGO/Virgo/KAGRA S2376: Identification of a GW compact binary merger candidate

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration report:

We identified the compact binary merger candidate S2376 during real-time
processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston
Observatory (L1) at 2025-05-21 05:53:59.754 UTC (GPS time: 1431842057.754). The
candidate was found by the Aframe [1] analysis pipeline.

S2376 is an event of interest because its false alarm rate, as estimated by the
online analysis, is 3.2e-08 Hz, or about one in a year. The event's properties
can be found at this URL:

https://gracedb.invalid/superevents/S2376

The classification of the GW signal, in order of descending probability, is BBH
(>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Assuming the candidate is astrophysical in origin, using an approximate
posterior, the probability that at least one of the compact objects is
consistent with a neutron star mass above one solar mass (HasNS) is <1%. [2]
Using the masses and assuming no spin the probability of matter outside the
final compact object (HasRemnant) is <1%. [2] HasRemnant is assumed to be zero
when the heavier component mass is below 1 solar mass. 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 <1%. The probability that the
lighter compact object is below 1 solar mass (HasSSM) is <1%.

The source chirp mass falls with highest probability in the bin (44.0, 88.0)
solar masses, assuming the candidate is astrophysical in origin.

One sky map is available at this time and can be retrieved from the GraceDB
event page:
 * amplfi.multiorder.fits,0, an initial localization generated by AMPLFI [3],
distributed via GCN and SCiMMA notices about 11 hours after the candidate event
time.

For the amplfi.multiorder.fits,0 sky map, the 90% credible region is 1778 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] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [3] Chatterjee et al. MLST 5, 045030 (2024) doi:10.1088/2632-2153/ad8982