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GCN Circular 25095

Subject
LIGO/Virgo S190718y: Upper limits from Fermi-GBM Observations
Date
2019-07-19T00:24:34Z (5 years ago)
From
Bagrat Mailyan at UAH <bm0054@uah.edu>
B. Mailyan (UAH), R. Hamburg (UAH) and A. Goldstein (USRA) report
on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo group:

For the LIGO/Virgo detection of GW trigger S190718y (GCN 25087) and using
the initial BAYESTAR skymap, Fermi-GBM was observing 24.0% of the
localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of S190718y.
An automated, blind search for short gamma-ray bursts below the onboard
triggering threshold in Fermi-GBM also identified no counterpart
candidates. The GBM
targeted search, the most sensitive, coherent search for GRB-like signals,
was run
from +/- 30 s around merger time, and also identified no counterpart
candidates.

Part of the LVC localization region is behind the Earth for Fermi, located
at RA = 15.0 and Dec = 11.4  with a radius of 67.3 degrees.
We therefore set upper limits on impulsive gamma-ray emission. Using the
representative soft, normal, and
hard GRB-like templates described in arXiv:1612.02395, we set the following
3 sigma flux upper limits over 10-1000 keV, weighted by GW localization
probability, (in units of 10^-8 erg/s/cm^2):

Timescale  soft     norm     hard
--------------------------------------
0.128 s:   7.2        16.       47.
1.024 s:   3.6        7.2       18.
8.192 s:   1.1        2.0       5.4

Assuming the median luminosity distance of 227 Mpc (z=0.051) from the GW
detection, we estimate the following intrinsic luminosity upper limits over
the 1 keV-10 MeV energy range (in units of 10^47 erg/s):

Timescale  soft     norm     hard
--------------------------------------
0.128 s:   6.84    14.2     68.4
1.024 s:   3.42    6.37     26.2
8.192 s:   1.04    1.77     7.86
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