GCN Circular 26334

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S191204r during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-12-04
17:15:26.092 UTC (GPS time: 1259514944.092). The candidate was found
by the GstLAL [1], CWB [2], MBTAOnline [3], and SPIIR [4] analysis
pipelines.

S191204r is an event of interest because its false alarm rate, as
estimated by the online analysis, is 3.1e-25 Hz, or about one in 1e17
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S191204r

At the time of the event the Virgo detector was performing calibration
injections, and was in low noise mode but not observing. At the exact
time of the event Virgo was between calibration injections and the
data was undisturbed. After discussion we agreed that the data was
usable for sky localization.

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%.

Three 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
[5], distributed via GCN notice about 42 minutes after the candidate
event time.
 * bayestar.fits.gz,1, an initial localization generated by BAYESTAR
[5], distributed via GCN notice about 54 minutes after the candidate
event time.
 * bayestar.fits.gz,2, an initial localization generated by BAYESTAR
[5], distributed via GCN notice about an hour after the candidate
event time.

The preferred sky map at this time is bayestar.fits.gz,2. For the
bayestar.fits.gz,2 sky map, the 90% credible region is 103 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 678 +/- 149 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] Adams et al. CQG 33, 175012 (2016)
 [4] Qi Chu, PhD Thesis, The University of Western Australia (2017)
 [5] Singer & Price PRD 93, 024013 (2016)

[GCN OPS NOTE (04dec19):  The typo extra "SUBJECT:" was removed from the SUBJECT:-line.]