GCN Circular 26454

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S191216ap during
real-time processing of data from LIGO Hanford Observatory (H1) and
Virgo Observatory (V1) at 2019-12-16 21:33:38.473 UTC (GPS time:
1260567236.473). The candidate was found by the GstLAL [1], MBTAOnline
[2], and PyCBC Live [3] analysis pipelines.

S191216ap is an event of interest because its false alarm rate, as
estimated by the online analysis, is 1.1e-23 Hz, or about one in 1e15
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S191216ap

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

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object has a mass < 3 solar masses (HasNS) is
19%. Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is
<1%.

One sky map is available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.fits.gz,0, an initial localization generated by BAYESTAR
[4], distributed via GCN notice about 16 minutes after the candidate
event time.

For the bayestar.fits.gz,0 sky map, the 90% credible region is 300
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 324 +/- 78 Mpc (a posteriori mean +/- standard
deviation).

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] Messick et al. PRD 95, 042001 (2017)
 [2] Adams et al. CQG 33, 175012 (2016)
 [3] Nitz et al. PRD 98, 024050 (2018)
 [4] Singer & Price PRD 93, 024013 (2016)