TITLE:   GCN CIRCULAR
NUMBER:  20840
SUBJECT: LIGO/Virgo G275404: Updated localization from LIGO data
DATE:    17/03/08 22:30:50 GMT
FROM:    Alex Urban at Caltech  <aurban@ligo.caltech.edu>

The LIGO Scientific Collaboration and Virgo report:

We have completed an initial Bayesian parameter estimation analysis of
the LIGO data around the time of the compact binary coalescence (CBC)
event candidate G275404 (GCN 20738) using LALInference (Veitch et al.,
PRD 91, 042003). This analysis used the IMRPhenomPv2 waveform
approximation, which includes the inspiral, merger, and ringdown
phases of the signal and accounts for the effects of spin precession.
We accounted for systematic errors due to uncertainty in the
calibration of the detectors' responses of up to 10% in amplitude and
10 degrees in phase. This follow-up analysis also used different, more
conservative assumptions on the detector noise properties.

The updated localization, LALInference.fits.gz, is available for
retrieval from the GraceDB event page:

https://gracedb.ligo.org/events/G275404 <https://gracedb.ligo.org/events/G275404>

The localization has arcs similar to the rapid BAYESTAR sky map, but
broader, and the conservative assumptions mentioned above result in
this case in a much larger area in the sky consistent with the
observed data. The 50% credible region spans 2000 deg2 and the 90%
credible region spans 17000 deg2 due to a diffuse component in the
probability distribution.

This parameter estimation analysis does not influence our estimation
of the significance of this event. It remains a marginal candidate
with a false alarm rate of about one in two months as reported in our
original Circular. If the signal is astrophysical, then the mass
estimates are consistent with a NSNS or NSBH binary.