GCN Circular 27358

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S200311bg during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2020-03-11
11:58:53.398 UTC (GPS time: 1267963151.398). The candidate was found
by the GstLAL [1], CWB [2], PyCBC Live [3], SPIIR [4], and MBTAOnline
[5] analysis pipelines.

S200311bg is an event of interest because its false alarm rate, as
estimated by the online analysis, is 8.9e-26 Hz, or about one in 1e18
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S200311bg

The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), MassGap
(<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
<1%. Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is
<1%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.fits.gz,0, an initial localization generated by BAYESTAR
[6], distributed via GCN notice about 2 minutes after the candidate
event time.
 * bayestar.fits.gz,1, an initial localization generated by BAYESTAR
[6], distributed via GCN notice about 10 minutes after the candidate
event time.

The preferred sky map at this time is bayestar.fits.gz,1. For the
bayestar.fits.gz,1 sky map, the 90% credible region is well fit by an
ellipse with an area of 52 deg2 described by the following DS9 region
(right ascension, declination, semi-major axis, semi-minor axis,
position angle of the semi-minor axis):
   icrs; ellipse(00h08m, -07d27m, 8.54d, 1.98d, 72.86d)
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 924 +/- 188 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] Klimenko et al. PRD 93, 042004 (2016)
 [3] Nitz et al. PRD 98, 024050 (2018)
 [4] Qi Chu, PhD Thesis, The University of Western Australia (2017)
 [5] Adams et al. CQG 33, 175012 (2016)
 [6] Singer & Price PRD 93, 024013 (2016)