# GCN Circular 35120

**Subject**

LIGO/Virgo/KAGRA S231119u: Identification of a GW compact binary merger candidate

**Date**

2023-11-19T08:26:52Z (a year ago)

**From**

valeria.sequino@na.infn.it

**Via**

Web form

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The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S231119u during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-19 07:52:48.095 UTC (GPS time: 1384415586.095). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], and SPIIR [4] analysis pipelines.
S231119u is an event of interest because its false alarm rate, as estimated by the online analysis, is 7.4e-08 Hz, or about one in 5 months. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S231119u
The classification of the GW signal, in order of descending probability, is BBH (95%), Terrestrial (5%), 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%. [5] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [5] 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 4%.
Two 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 [6], distributed via GCN notice about 27 seconds after the candidate event time.
* bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 5 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 4240 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 8237 +/- 2603 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] Chu et al. PRD 105, 024023 (2022)
[5] Chatterjee et al. ApJ 896, 54 (2020)
[6] Singer & Price PRD 93, 024013 (2016)
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