TITLE: GCN CIRCULAR NUMBER: 21474 SUBJECT: LIGO/Virgo G297595: Identification of a GW Binary Merger Candidate DATE: 17/08/14 12:28:42 GMT FROM: Erik Katsavounidis at MIT The LIGO Scientific Collaboration and the Virgo Collaboration report: The gstlal CBC analysis (Messick et al. Phys. Rev. D 95, 042001, 2017) identified candidate G297595 during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2017-08-14 10:30:43 UTC (GPS time: 1186741861.5268). G297595 is an event of interest because its false alarm rate, as determined by the online analysis, is ~1/80000 years, passing our stated alert threshold of ~1/month. The event's properties can be found at this URL: https://gracedb.ligo.org/events/G297595 This event was also identified in real-time by another CBC pipeline, PyCBC (Nitz, et al. 2017, arxiv 1705.01513), and the unmodeled burst pipelines, cWB (Klimenko et al. Phys. Rev. D 93, 042004 (2016)) and LIB (Lynch et al. Phys. Rev. D 95, 104046 (2017)). 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. For more details on the source classification, please consult this technical document: https://dcc.ligo.org/T1600571/public/main . This is among the first GW event candidate detected in coincidence with Advanced LIGO and Advanced Virgo, and the first event candidate that registered loudly enough in Virgo to contribute significantly to position reconstruction. A rapid localization with distance information generated by the BAYESTAR pipeline (e.g., Singer et al. 2016, ApJL 829, 15) is available at this time and can be retrieved from the GraceDB event page: bayestar.fits.gz, using data from LIGO Hanford, LIGO Livingston, and Virgo, with probability concentrated in a roughly elliptical region centered at R.A.=02h44m, Dec.-45d29m. The 50% credible region spans about 22 deg2 and the 90% region about 97 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 550 +/- 130 Mpc. For comparison, using data from LIGO Hanford and LIGO Livingston alone, the 50% credible region spanned 333 deg2 and the 90% area 1158 deg2. Updates on our analysis of this event, including updated localizations will be sent as they become available.