TITLE:   GCN CIRCULAR
NUMBER:  25122
SUBJECT: LIGO/Virgo S190720a: Upper limits from Fermi-GBM Observations
DATE:    19/07/20 12:31:57 GMT
FROM:    Joshua Wood at MSFC/Fermi-GBM  <joshua.r.wood@nasa.gov>

J. Wood (NASA/MSFC) reports on behalf of the Fermi-GBM Team and the GBM-LIGO/Virgo group:

For the LIGO/Virgo detection of GW trigger S190720a (GCN 25115) and using the initial BAYESTAR skymap, Fermi-GBM was observing 73.2% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of S190720a. 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 templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV (in units of 10^-8 erg/s/cm^2):

Timescale  Soft   Normal   Hard
------------------------------------
0.128 s:   37.    57.      110.
1.024 s:   13.    17.      40.
8.192 s:   3.2    4.9      9.5 

Assuming the median luminosity distance of 1071 Mpc (z=0.215) from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^48 erg/s):

Timescale  Soft   Normal   Hard
------------------------------------
0.128 s:   78.6   110.     352.
1.024 s:   27.6   32.7     128.
8.192 s:   6.80   9.42     30.4