TITLE:   GCN CIRCULAR
NUMBER:  34235
SUBJECT: LIGO/Virgo/KAGRA S230723ac: Identification of a GW compact binary merger candidate
DATE:    23/07/23 11:55:31 GMT
FROM:    ricardo.cabrita@uclouvain.be

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

We identified the compact binary merger candidate S230723ac during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-07-23 10:18:34.451 UTC (GPS
time: 1374142732.451). The candidate was found by the, GstLAL
[1], MBTA [2], and SPIIR [3] analysis pipelines.


S230723ac is an event of interest because its false alarm rate, as
estimated by the online analysis, is 5.3e-08 Hz, or about one in 7
months. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230723ac

The classification of the GW signal, in order of descending
probability, is BBH (87%), Terrestrial (13%), 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%. [4] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [4] 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 [5], distributed via GCN notice about 25 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [5], 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
1281 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 1686 +/- 515 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] Tsukada et al. arXiv:2305.06286 (2023) and Ewing et al. arXiv:2305.05625 (2023)
 [2] Aubin et al. CQG 38, 095004 (2021)
 [3] Chu et al. PRD 105, 024023 (2022)
 [4] Chatterjee et al. ApJ 896, 54 (2020)
 [5] Singer & Price PRD 93, 024013 (2016)