GCN Circular 34494

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

We identified the compact binary merger candidate S230819ax during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-08-19 17:19:10.455 UTC (GPS
time: 1376500768.455). The candidate was found by the CWB [1], GstLAL
[2], MBTA [3], oLIB [4], and SPIIR [5] analysis pipelines.

S230819ax is an event of interest because its false alarm rate, as
estimated by the online analysis, is 8.8e-09 Hz, or about one in 3
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230819ax

The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS
(<1%).

There were noise transients at the LIGO Hanford and evidence for 
nonstationary noise at both LIGO detectors which may affect the 
parameters or the significance of the candidate.

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,1, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 25 seconds after the
candidate event time.
 * bayestar.multiorder.fits,2, 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,2. For
the bayestar.multiorder.fits,2 sky map, the 90% credible region is
4126 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 4872 +/- 1527 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
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [3] Aubin et al. CQG 38, 095004 (2021)
 [4] Lynch et al. PRD 95, 104046 (2017)
 [5] Chu et al. PRD 105, 024023 (2022)
 [6] Chatterjee et al. ApJ 896, 54 (2020)
 [7] Singer & Price PRD 93, 024013 (2016)