TITLE:   GCN CIRCULAR
NUMBER:  35177
SUBJECT: LIGO/Virgo/KAGRA S231118an: Updated Sky localization and EM Bright Classification
DATE:    23/11/24 16:13:46 GMT
FROM:    Sylvia Biscoveanu at MIT <sylvia.biscoveanu@ligo.org>

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

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S231118an (GCN Circular 35096). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline1.multiorder.fits, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S231118an

Based on posterior support from parameter estimation [1], under the assumption that the candidate S231118an 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 21%.

For the Bilby.offline1.multiorder.fits sky map, the 90% credible region is 1107 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1337 +/- 347 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] Ashton et al. ApJS 241, 27 (2019) and Morisaki et al. arXiv:2307.13380 (2023)
 [2] Chatterjee et al. ApJ 896, 54 (2020)