TITLE:   GCN CIRCULAR
NUMBER:  24642
SUBJECT: LIGO/Virgo S190521r: Upper limits from Fermi-GBM observations
DATE:    19/05/21 17:45:13 GMT
FROM:    Suraj Poolakkil at UAH  <sp0076@uah.edu>

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

For S190521r, and using the initial BAYESTAR skymap, Fermi-GBM was
observing 100.0% 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 S190521r (GCN 24632). 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, weighted by the remaining visible GW localization probability
(in units of erg/s/cm^2):

Timescale  soft     norm     hard
--------------------------------------
0.128 s:   6.5e-07  8.4e-07  2.1e-06
1.024 s:   1.6e-07  2.4e-07  4.3e-07
8.192 s:   5.8e-08  7.8e-08  1.5e-07

Assuming the median luminosity distance of ~1136 Mpc from the GW detection,
we estimate intrinsic luminosity upper limits of (1.4-16.1)E49 erg/s for the
soft template, (1.7-18.8)E49 erg/s for the normal template, and
(5.5-78.2)E49 erg/s for the hard template over the 1 keV-10 MeV energy
range.