GCN Circular 33944

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

We identified the compact binary merger candidate S230609u during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-06-09 06:49:58.273 UTC (GPS
time: 1370328616.273). The candidate was found by the CWB [1], MBTA
[2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S230609u is an event of interest because its false alarm rate, as
estimated by the online analysis, is 1e-08 Hz, or about one in 3
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230609u

The classification of the GW signal, in order of descending
probability, is BBH (96%), Terrestrial (4%), 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 any one of the binary components lie between 3 to 5 solar mass
(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 26 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
1918 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 2799 +/- 842 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] Aubin et al. CQG 38, 095004 (2021)
 [3] Tsukada et al. arXiv:2305.06286
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  (2023) and Ewing et al.
arXiv:2305.05625
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  (2023)
 [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)