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GCN Circular 18858

Subject
LIGO/Virgo G184098: Refined localizations from CBC parameter estimation
Date
2016-01-13T20:15:04Z (8 years ago)
From
Leo Singer at NASA/GSFC <leo.p.singer@nasa.gov>
The LIGO Scientific Collaboration and Virgo report:

We have completed a Bayesian parameter estimation analysis of the GW
candidate G184098 (GCN 18330) under the assumption that the signal
arises from a compact binary coalescence (CBC) and using the latest
offline calibration of the GW strain data (GCN 18851). The data is
still found to be most consistent with a binary black hole merger
(GCN 18388).

Two refined sky maps are now available and can be retrieved from
GraceDB (https://gracedb.ligo.org/events/G184098):

 * bayestar_gstlal_C01.fits.gz, produced by rapid triangulation of
   times, phases, and amplitudes on arrival. It was not available in
   low latency because at the time of the event our online CBC template
   banks were configured for neutron star but not binary black hole
   mergers. This localization is based upon the recovery of the CBC
   candidate from the offline GSTLAL search (GCN 18851), though it is
   practically interchangeable with the BAYESTAR sky map based on the
   PyCBC recovery.

 * LALInference_skymap.fits.gz, using Bayesian Markov-chain Monte Carlo
   and nested sampling to perform forward modeling of the full GW
   signal including spin precession and regression of systematic
   calibration errors. We regard the LALInference sky map as the most
   accurate and *authoritative* localization for this event. Though
   additional parameter estimation runs with different waveform
   approximants are ongoing, we expect that they will agree closely
   with this one.

Both of the sky maps above agree with the initial LALinference Burst
(LIB) localization (GCN 18330, LIB_skymap.fits.gz) on favoring the
southern portion of the annulus determined by an arrival time
difference between LIGO Hanford and LIGO Livingston of about 7 ms.

The table below presents a quantitative comparison of the available
localizations along the lines of Sec. 4.5 of Essick et al. (2015,
http://adsabs.harvard.edu/abs/2015ApJ...800...81E). The first column
gives the area in deg2 of the 90% credible region, and the second
column gives the area in deg2 of the overlap with the LALInference 90%
credible region.

 Area  Overlap   Algorithm (filename)         
-------------------------------------------------------------
  310    230     cWB (skyprobcc_cWB_complete.fits)
  750    270     LIB (LIB_skymap.fits.gz)
  400    350     BAYESTAR (bayestar_gstlal_C01.fits.gz)
  600     -      LALInference (LALInference_skymap.fits.gz)
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