LIGO/Virgo/KAGRA S231028bg

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

We identified the compact binary merger candidate S231028bg during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-10-28 15:30:06.253 UTC (GPS time: 1382542224.253). The candidate was found by the CWB [1], GstLAL [2], PyCBC Live [3], and SPIIR [4] analysis pipelines.

S231028bg is an event of interest because its false alarm rate, as estimated by the online analysis, is 7.6e-31 Hz, or about one in 1e23 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S231028bg

The classification of the GW signal, in order of descending probability, is BBH (>99%), NSBH (<1%), BNS (<1%), or Terrestrial (<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 <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 [6], distributed via GCN notice about 26 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 1179 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 3695 +/- 945 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
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  (2023)
 [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)

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 S231028bg (GCN Circular 34895
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 ). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S231028bg

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1207 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4221 +/- 923 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
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  (2023)