TITLE:   GCN CIRCULAR
NUMBER:  34904
SUBJECT: LIGO/Virgo/KAGRA S231029y: Identification of a GW compact binary merger candidate
DATE:    23/10/29 14:08:36 GMT
FROM:    Marco Drago at Sapienza University and INFN Roma 1 <marco.drago@ligo.org>

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

We identified the compact binary merger candidate S231029y during real-time processing of data from LIGO Livingston Observatory (L1) at 2023-10-29 11:15:08.754 UTC (GPS time: 1382613326.754). The candidate was found by the GstLAL [1] analysis pipeline.

S231029y is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231029y

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%.

Three sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 33 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 5 minutes after the candidate event time.
 *  Bilby.multiorder.fits,0, an updated skymap from further analysis of L1 data around the time of the merger performed using Bilby [4] also distributed via GCN Notice.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 29973 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3292 +/- 1313 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] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625 (2023)
 [2] Chatterjee et al. ApJ 896, 54 (2020)
 [3] Singer & Price PRD 93, 024013 (2016)
 [4] Ashton et al. ApJS 241, 27 (2019) and Morisaki et al. arXiv:2307.13380 (2023)