TITLE:   GCN CIRCULAR
NUMBER:  35168
SUBJECT: LIGO/Virgo/KAGRA S231123cg: Identification of a GW compact binary merger candidate
DATE:    23/11/23 15:40:58 GMT
FROM:    cervane.grimaud@lapp.in2p3.fr

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

We identified the compact binary merger candidate S231123cg during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-23 13:54:30.634 UTC (GPS time: 1384782888.634). The candidate was found by the CWB [1], GstLAL [2], and PyCBC Live [3] analysis pipelines.

S231123cg is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231123cg

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 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%.

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 notice about 35 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [5], 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 2714 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1148 +/- 338 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/userguide/.

 [1] Klimenko et al. PRD 93, 042004 (2016)
 [2] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625 (2023)
 [3] Dal Canton et al. ApJ 923, 254 (2021)
 [4] Chatterjee et al. ApJ 896, 54 (2020)