TITLE: GCN CIRCULAR NUMBER: 34775 SUBJECT: LIGO/Virgo/KAGRA S230927be: Identification of a GW compact binary merger candidate DATE: 23/09/27 16:18:47 GMT FROM: brina.martinez@ligo.org

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

We identified the compact binary merger candidate S230927be during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-09-27 15:38:32.919 UTC (GPS time: 1379864330.919). The candidate was found by the CWB [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S230927be is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230927be

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<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%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] 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 [7], distributed via GCN notice about 35 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], 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 527 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1056 +/- 308 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. arXiv:2305.06286 (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] Chatterjee et al. ApJ 896, 54 (2020) [7] Singer & Price PRD 93, 024013 (2016)

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