TITLE:   GCN CIRCULAR
NUMBER:  35330
SUBJECT: LIGO/Virgo/KAGRA S231213ap: Identification of a GW compact binary merger candidate
DATE:    23/12/13 16:00:09 GMT
FROM:    Lorenzo Mirasola at Università di Cagliari/INFN Cagliari <lorenzo.mirasola@ca.infn.it>

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

We identified the compact binary merger candidate S231213ap during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-12-13 11:14:17.914 UTC (GPS time: 1386501275.914). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S231213ap is an event of interest because its false alarm rate, as estimated by the online analysis, is 6.3e-10 Hz, or about one in 50 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231213ap

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).
After parameter estimation by RapidPE-RIFT [6], the updated classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS (0%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] 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 [8], distributed via GCN notice about 40 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], 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 2106 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 2863 +/- 856 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] Aubin et al. CQG 38, 095004 (2021)
 [4] Dal Canton et al. ApJ 923, 254 (2021)
 [5] Chu et al. PRD 105, 024023 (2022)
 [6] Rose et al. arXiv:2201.05263 (2022) and Pankow et al. PRD 92, 023002 (2015)
 [7] Chatterjee et al. ApJ 896, 54 (2020)
 [8] Singer & Price PRD 93, 024013 (2016)