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GCN Circular 21656

Subject
LIGO/Virgo G298936: Identification of a GW Binary Black Hole Candidate
Date
2017-08-23T14:07:00Z (7 years ago)
From
Leo Singer at NASA/GSFC <leo.p.singer@nasa.gov>
The LIGO Scientific Collaboration and Virgo report:

The gstlal CBC analysis (Messick et al. Phys. Rev. D 95, 042001, 2017)
identified candidate G298936 during real-time processing of data from LIGO
Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at
2017-08-23 13:13:58.518 UTC (GPS time: 1187529256.518). The Virgo (V1)
detector was online but was not included in the analysis because it had
recently re-acquired lock. We are investigating the quality of the V1 data
to determine whether it can be used.

G298936 is an event of interest because its false alarm rate, as
determined by the online analysis, is 1.739e-11 Hz, or about 1 in 1800
years. The event's properties can be found at this URL:

https://gracedb.ligo.org/events/G298936

This event was also identified in real-time by two other CBC pipelines,
PyCBC (Nitz, et al. 2017, arXiv:1705.01513) and MBTA (Adams et al. CQG 33,
175012, 2016). It was also detected by 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, 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 .

A rapid localization with distance information generated by the BAYESTAR
pipeline (e.g., Singer et al. 2016, ApJL 829, 15) is available and can be
retrieved from the GraceDB event page: bayestar.fits.gz.

The 50% credible region spans about 605 deg2 and the 90% region about 2145
deg2. The probability is concentrated in a pair of long, thin arcs that
spread across both the northern and southern hemispheres. Marginalized
over the whole sky, the a posteriori luminosity distance estimate is 1387
+/- 414 Mpc. It may be possible to improve the localization if the V1 data
can be included.

Updates on our analysis of this event, including updated localizations
will be sent as they become available.
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