TITLE: GCN CIRCULAR NUMBER: 21693 SUBJECT: LIGO/Virgo G299232: Identification of a GW Compact Binary Coalescence Candidate DATE: 17/08/25 14:25:50 GMT FROM: Karelle Siellez at Georgia Inst of Tech The LIGO Scientific Collaboration and Virgo report: The MBTA CBC analysis (Adams et al. CQG 33, 175012, 2016) identified candidate G299232 during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2017-08-25 13:13:31 UTC (GPS time: 1187702035.9831). Data from Virgo (V1) were included in a followup analysis using the PyCBC toolkit (Nitz, et al. 2017, arXiv:1705.01513) and used to generate the localization given below. G299232 is a low-significance CBC candidate with a false alarm rate, As determined by the online analysis, of 1.68e-07 Hz, or about 5.3 per year, just within our threshold of 6 per year for announcing candidates. The event's properties can be found at this URL: https://gracedb.ligo.org/events/view/G299232 If the candidate is astrophysical in origin, it appears consistent with the merger of a black hole and a neutron star. For more details on the source classification, please consult this technical document: https://dcc.ligo.org/T1600571/public/main A rapid localization with distance information generated by the BAYESTAR pipeline (Singer et al. 2016, ApJL 829, 15) including information from H1, L1, and V1 is available and can be retrieved from the GraceDB event page: bayestar-HLV.fits.gz. The 50% credible region spans about 450 deg2 and the 90% region about 2040 deg2. The probability is concentrated in a pair of long, thin arcs that spread across both the northern and southern hemispheres, although the probability is concentrated in the northern hemisphere. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 339 +/-110 Mpc. Updates on our analysis of this event, including updated localizations will be sent as they become available.