GCN Circular 21221

The LIGO Scientific Collaboration and Virgo Collaboration report:

The PyCBC binary-merger search (Nitz et al., arXiv:1705.01513
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 ; Usman
et al. 2016, CQG 33, 215004) identified candidate G288732 in analysis
of data from the LIGO Livingston Observatory (L1) at 2017-06-08
02:01:16.492 UTC (GPS time: 1180922494.492). This circular was delayed
as the LIGO Hanford detector was not in observing mode at this time
and manual investigations were required.

The Livingston detector was operating nominally at the time of the
trigger, however the Hanford detector was undergoing a period of
active commissioning at the time of the trigger. The trigger was first
identified by inspection of PyCBC triggers from the Livingston
data. Manual follow-up of the Hanford data identified a coincident
trigger with consistent parameters.  The Virgo detector was locked but
in a commissioning mode at the time; Virgo data has not yet been used
in this analysis. Offline analysis is ongoing.

Commissioning activities were ongoing for two minutes around the time
of the trigger, but studies suggest this activity only affected
frequencies below 30 Hz. This is below the starting frequency of the
manual re-analysis. Other than the Hanford activity below 30 Hz, the
state and calibration of the detectors appears nominal.

Due to the Hanford commissioning activities, the amount of data
available to estimate the noise background was limited to
approximately 15 minutes. Using these data, we estimate that the false
alarm rate of G288732 is less than (i.e. more significant than) 2.6
per year, however the signal-to-noise ratio of this candidate is
consistent with a higher significance.

The event's properties can be found at this URL:
https://gracedb.ligo.org/events/G288732

If G288732 is astrophysical, it is likely to be a binary black hole
merger.

The gstlal binary-merger search (Messick et al., 2016, PRD 95, 042001)
identified a consistent trigger in the Livingston data at the time of
the candidate.

One sky map with distance information (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, an initial localization generated by the BAYESTAR
pipeline. The 50% credible region spans about 230 deg2 and the 90% region
about 860 deg2. The luminosity distance is 320 +/- 100 Mpc (all-sky mean
+/- standard deviation). This is the preferred sky map at this time.

We caution that the parameters of this candidate may be subject to change
as data-quality and calibration studies are ongoing.