[vsnet-grb-info 22149] LIGO/Virgo S190426c: Fermi GBM Observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24248
SUBJECT: LIGO/Virgo S190426c: Fermi GBM Observations
DATE:    19/04/26 20:50:31 GMT
FROM:    Adam Goldstein at Fermi-GBM, USRA  <adam.michael.goldstein at gmail.com>

C. Fletcher (USRA) reports on behalf of the Fermi-GBM Team and the
GBM-LIGO/Virgo group:

For S190426c, and using the updated BAYESTAR skymap, Fermi-GBM was
observing 100% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the
LIGO/Virgo detection of GW trigger S190426c (GCN 24237). 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.

We therefore set upper limits on impulsive gamma-ray emission.  Using the
representative soft, normal, and hard GRB-like spectral templates described
in arXiv:1612.02395, we set the following 3 sigma flux upper limits over
10-1000 keV (in units of 10^-7 erg/s/cm^2):

Timescale  Soft     Normal   Hard
------------------------------------
0.1 s:     3.9-7.0  8.3-12.  26.-30.
1.0 s:     1.2-2.2  2.6-3.7  7.7-8.8
10  s:     0.4-0.7  0.8-1.1  2.4-2.7

Assuming the mean luminosity distance of ~375 Mpc from the GW detection, we
estimate intrinsic luminosity upper limits of (0.09-1.6)E49 erg/s for the
soft template, (0.2-2.6)E49 erg/s
for the normal template, and (0.9-10.6)E49 erg/s for the hard template over
the 1 keV-10 MeV energy range.