TITLE:   GCN CIRCULAR
NUMBER:  25465
SUBJECT: Fermi GBM-190816: Updated localization of subthreshold GRB-GW candidate
DATE:    19/08/24 16:42:36 GMT
FROM:    Peter Shawhan at U of Maryland/LSC  <pshawhan@umd.edu>

The LIGO Scientific Collaboration, the Virgo Collaboration, and the
Fermi GBM team report:

LSC and Virgo members have conducted further analysis of the LIGO and
Virgo data around the time of the possible compact binary coalescence
(CBC) candidate described in GCN Circular 25406. Parameter estimation
has been performed using LALInference [1] to produce a new sky map.

Fermi GBM data has also been re-analyzed utilizing the GBM targeted
search [2,3] on finer timescales to optimize the source window for a
refined localization. The current preferred GBM localization is
broadly consistent with the original localization, but the highest
probability region has shifted.

A new joint sky map has been produced. The joint localization is
concentrated in similar areas of the sky but the high-probability
regions have shifted significantly. The 90% error region area is 3219
sq. deg. while the 50% error region area is 744 sq. deg. This sky map
is available with the file name GBM-190816-with-LV-v2.fits on the
OpenLVEM wiki page at

https://wiki.gw-astronomy.org/OpenLVEM/FermiGBM-LVC

There is no change at this time in our assessment of the significance
of either the potential short GRB or the GW trigger. Both remain
sub-threshold individually; the estimated joint false alarm rate is on
the order of the usual LIGO-Virgo public alert threshold of 1 per 2
months.

[1] Veitch, et al. PRD 91, 042003 (2015)
[2] Blackburn et al. 2015, ApJS 217, 8
[3] Goldstein et al. arXiv:1903.12597