[vsnet-grb-info 22542] LIGO/Virgo S190521g : Upper limits from Fermi-GBM observations

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24641
SUBJECT: LIGO/Virgo S190521g : Upper limits from Fermi-GBM observations
DATE:    19/05/21 17:44:12 GMT
FROM:    Suraj Poolakkil at UAH  <sp0076 at uah.edu>

S. Poolakkil (UAH) and A. Goldstein (USRA) report on behalf of the
Fermi-GBM Team and the GBM-LIGO/Virgo group:

For S190521g, and using the updated LALInference skymap, Fermi-GBM was
observing 70.9% 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 S190521g (GCN 24621). 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 = 172.7 and Dec = -25.4 with a radius of 67.5 degrees.
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, weighted by the remaining visible GW localization probability
(in units of erg/s/cm^2):

Timescale  soft     norm     hard
--------------------------------------
0.128 s:   7.6e-07  9.7e-07  1.8e-06
1.024 s:   2.6e-07  3.4e-07  5.2e-07
8.192 s:   6.7e-08  1.1e-07  1.9e-07

Assuming the median luminosity distance of ~3931 Mpc from the GW detection,
we estimate intrinsic luminosity upper limits of (1.9-22.5)E50 erg/s for the
soft template, (2.7-24.5)E50 erg/s for the normal template, and
(8.1-77.3)E50 erg/s for the hard template over the 1 keV-10 MeV energy
range.