TITLE:   GCN CIRCULAR
NUMBER:  33818
SUBJECT: LIGO/Virgo/KAGRA S230518h: Upper limits from Fermi-GBM Observations
DATE:    23/05/18 20:36:37 GMT
FROM:    Sarah Dalessi at UAH <sd0104@uah.edu>

S. Dalessi (UAH) and C. Fletcher (USRA) report on behalf of the
Fermi-GBM Team and the GBM-LIGO/Virgo/KAGRA group

For S230518h and using the updated Bilby skymap (GCN 33816), Fermi-GBM
was observing 48.9% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the
LIGO/Virgo/KAGRA (LVK) detection of GW trigger S230518h (GCN 33813).
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 LVK localization region is behind the Earth for Fermi,
located at an RA=32.3, Dec=-3.6 with a radius of 67.6 degrees. We
therefore set upper limits on impulsive gamma-ray emission for the LVK
Consortia 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   Normal   Hard
------------------------------------
0.128 s:     1.4   2.7     6.1
1.024 s:     0.4   0.8     2.0
8.192 s:     0.1   0.3     0.8

Assuming the median luminosity distance of  277.9  Mpc from the GW
detection, we estimate the following intrinsic luminosity upper limits
over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft     Normal   Hard
------------------------------------
0.128s: 0.020      0.035     0.132
1.024s: 0.006      0.010     0.043
8.192s: 0.002      0.004     0.018