GCN Circular 25814

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190923y during real-time
processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston
Observatory (L1) at 2019-09-23 12:55:59.646 UTC (GPS time: 1253278577.646).
The candidate was found by the PyCBC Live [1], MBTAOnline [2], and GstLAL
[3] analysis pipelines.

S190923y is an event of interest because its false alarm rate, as estimated
by the online analysis, is 4.8e-08 Hz, or about one in 7 months. The
event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S190923y

The classification of the GW signal, in order of descending probability, is
NSBH (68%), Terrestrial (32%), BNS (<1%), BBH (<1%), or MassGap (<1%).

Assuming the candidate is astrophysical in origin, there is strong evidence
for the lighter compact object having a mass < 3 solar masses (HasNS:
>99%). Using the masses and spins inferred from the signal, there is strong
evidence against matter outside the final compact object (HasRemnant: <1%).

One sky map is available at this time and can be retrieved from the GraceDB
event page:
* bayestar.fits.gz, an updated localization generated by BAYESTAR [4],
distributed via GCN notice about 6 minutes after the candidate

For the bayestar.fits.gz sky map, the 90% credible region is 2107 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 438 +/- 133 Mpc (a posteriori mean +/- standard deviation).

The Virgo (V1) detector was not in observing mode at the time of the
candidate but was collecting low-noise data during a tuning of the
interferometer working point.  We are investigating whether the V1 data can
be used in parameter estimation analyses.

For further information about analysis methodology and the contents of this
alert, refer to the LIGO/Virgo Public Alerts User Guide <
https://emfollow.docs.ligo.org/userguide/>.

 [1] Nitz et al. PRD 98, 024050 (2018)

 [2] Adams et al. CQG 33, 175012 (2016)

 [3] Messick et al. PRD 95, 042001 (2017)

 [4] Singer & Price PRD 93, 024013 (2016)