LIGO/Virgo/KAGRA S230709bi

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

We identified the compact binary merger candidate S230709bi during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-07-09 12:27:27.202 UTC (GPS
time: 1372940865.202). The candidate was found by the CWB [1], MBTA
[2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S230709bi is an event of interest because its false alarm rate, as
estimated by the online analysis, is 3.1e-09 Hz, or about one in 10
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S230709bi

The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), 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%. [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 25 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 14 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
2937 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 5009 +/- 1547 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)

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

We have conducted further offline analysis of the LIGO Hanford Observatory
(H1) and LIGO Livingston Observatory (L1) data around the time of the
compact binary merger (CBC) candidate S230709bi (GCN Circular 34175
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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/S230709bi

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is
2644 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 4364 +/- 1585 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)