TITLE: GCN CIRCULAR NUMBER: 41404 SUBJECT: LIGO/Virgo/KAGRA S250818t: Identification of a GW compact binary merger candidate DATE: 25/08/18 02:52:38 GMT FROM: Jiyoon Sun jiyoon.sun.h@gmail.com

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

We identified the compact binary merger candidate S250818t during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 02:08:58.392 UTC (GPS time: 1439518156.392). The candidate was found by the GstLAL [1], MBTA [2], and PyCBC Live [3] analysis pipelines.

S250818t is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.7e-11 Hz, or about one in 1e3 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250818t

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, the probability that at least one of the compact objects 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 for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 2%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses, assuming the candidate is astrophysical in origin.

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 and SCiMMA notices about 32 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], distributed via GCN and SCiMMA notices 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 377 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1311 +/- 350 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 and Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [2] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234 [3] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [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

View this GCN Circular online at https://gcn.nasa.gov/circulars/41404. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...