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GCN Circular 35096

LIGO/Virgo/KAGRA S231118an: Identification of a GW compact binary merger candidate
2023-11-18T09:43:30Z (5 months ago)
Annalisa Allocca at Università di Napoli Federico II <>
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The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S231118an during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-11-18 09:06:02.009 UTC (GPS time: 1384333580.009). The candidate was found by the GstLAL [1], MBTA [2], PyCBC Live [3], and SPIIR [4] analysis pipelines.

S231118an is an event of interest because its false alarm rate, as estimated by the online analysis, is 7.5e-08 Hz, or about one in 5 months. The event's properties can be found at this URL:

The classification of the GW signal, in order of descending probability, is BBH (74%), Terrestrial (24%), NSBH (1%), or BNS (<1%).

For this candidate, the classification of the GW signal is biased such that the probability of NSBH (1.4%) and BNS (0%) are overestimated while that of Terrestial (24%) is underestimated. The bug is being fixed. Astronomers are advised to keep this in mind while deciding to follow this event.

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 2%.

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 [6], distributed via GCN notice about 27 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, 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,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 1048 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1396 +/- 377 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

 [1] Tsukada et al. PRD 108, 043004 (2023) and Ewing et al. arXiv:2305.05625 (2023)
 [2] Aubin et al. CQG 38, 095004 (2021)
 [3] Dal Canton et al. ApJ 923, 254 (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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