TITLE: GCN CIRCULAR NUMBER: 18728 SUBJECT: LIGO/Virgo G211117: Identification of a GW CBC Candidate DATE: 15/12/27 17:39:45 GMT FROM: Leo Singer at NASA/GSFC 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.