TITLE: GCN CIRCULAR NUMBER: 21431 SUBJECT: LIGO/Virgo G296853: Identification of a GW Binary Merger Candidate DATE: 17/08/09 10:29:45 GMT FROM: Reed Clasey Essick at MIT The LIGO Scientific Collaboration and the Virgo Collaboration report: The gstlal CBC analysis identified candidate G296853 during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2017-08-09 08:28:21.747 UTC (GPS time: 1186302519.747). Virgo (V1) was observing at the time, but with a range only one quarter of that of L1, and data from V1 was not used when estimating this event’s significance. G296853 is an event of interest because its false alarm rate, as determined by the online analysis, is 7.51e-09 Hz or about one in 4 years, passing our stated alert threshold of ~1/month. The event's properties can be found at this URL: https://gracedb.ligo.org/events/G296853 This event was also identified in real-time by two other pipelines: Coherent WaveBurst (cWB) and Multi-Band Template Analysis (MBTA). A sky map is available at this time and can be retrieved from the GraceDB event page: * bayestar.fits.gz, a localization generated by BAYESTAR, distributed via GCN notice about 40 minutes after the event. At this time, the BAYESTAR map only uses information from H1 and L1. The 50% credible region spans about 316 deg2 and the 90% region about 1155 deg2, and the posterior probability is mostly confined to the Southern Hemisphere. The mean posterior distance is about 1 Gpc. The event appears consistent with the merger of two black holes at this time, and there is little chance either component was a neutron star. Updates on our analysis of this event, including updated localizations which include Virgo data, will be sent as they become available.