GCN Circular 21431

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.