GCN Circular 21474

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.