GCN Circular 18728

The LIGO Scientific Collaboration and Virgo report:

The online gstlal CBC analysis, which is sensitive to binary coalescence
events from systems containing neutron stars and/or black holes,
identified candidate G211117 during real-time processing of data from
LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at
2015-12-26 03:38:53.648 UTC (GPS time: 1135136350.648).

The candidate was identified by an expanded low-latency pipeline
configuration that is sensitive to stellar-mass BNS, NSBH, and BBH
mergers. G211117 is an event of interest because its false alarm rate, as
determined by the online analysis, passed our stated alert threshold of
~1/month. The event's properties can be found at this URL:

https://gracedb.ligo.org/events/G211117

If confirmed as astrophysical, the system contains at least one and most
likely two black holes.

The candidate was below the threshold for detection by the low-latency
un-modeled burst searches. However, manual offline analysis with Coherent
WaveBurst (cWB) recovered a candidate with consistent timing and
amplitude, designated G211182:

https://gracedb.ligo.org/events/G211182

Two sky maps are available at this time and can be retrieved from the
GraceDB event pages:

 * bayestar.fits.gz, an initial localization generated by the BAYESTAR
   pipeline based on the CBC analysis about 2 minutes after the event. The
   probability is concentrated in two long, thin arcs. The 50% credible
   region spans about 430 deg2 and the 90% region about 1400 deg2. This is
   the preferred sky map at this time.

 * skyprobcc_cwb.fits, generated from the cWB offline analysis about six
   hours after the event. The area supported by this sky map is
   qualitatively very similar to and essentially a superset of the
   BAYESTAR sky map, with a 90% credible area of 2200 deg2.