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GCN Circular 25196

Subject
LIGO/Virgo S190728q: Upper limits from Fermi-GBM Observations
Date
2019-07-28T13:09:00Z (5 years ago)
From
Rachel Hamburg at UAH <rkh0007@uah.edu>
R. Hamburg (UAH) reports on behalf of the Fermi-GBM Team and the
GBM-LIGO/Virgo group:

For the LIGO/Virgo detection of GW trigger S190728q (GCN 25187) and using
the triple-detector BAYESTAR skymap, Fermi-GBM was observing 84.1% of the
localization probability at event time.

Approximately 15 min before the S190728q event, GBM triggered onboard
to the short gamma-ray burst GRB 190728A (GCN 25181). However based
on the large temporal offset and the difference in localization, this
trigger is
not considered a likely counterpart.

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 = 129.9 and Dec = -11.1 with a radius of 67.5 degrees.
We therefore set upper limits on impulsive gamma-ray emission for
the LVC localization region visible to Fermi at merger time. 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^-7 erg/s/cm^2):

Timescale  soft   norm   hard
--------------------------------------
0.128 s:    5.3   6.0   15.0
1.024 s:    2.0   2.7    4.2
8.192 s:    0.6   0.8    1.1

Assuming the median luminosity distance of 795 Mpc (z=0.165) from the GW
detection, we estimate the following intrinsic luminosity upper limits over
the 1 keV-10 MeV energy range (in units of 10^49 erg/s):

Timescale  soft   norm   hard
--------------------------------------
0.128 s:   6.2    6.4    27.0
1.024 s:   2.3    2.9    7.6
8.192 s:   0.7    8.6    2.0
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