GCN Circular 24082

N. Omodei (Stanford Univ.), F. Longo (University and INFN Trieste), M.
Arimoto (Kanazawa Univ.),
and E. Bissaldi (Politecnico & INFN Bari) report on behalf of the
Fermi-LAT Collaboration:

We have searched data collected by the Fermi Large Area Telescope (LAT)
for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal
coincidence with the LIGO/Virgo trigger ST190408an (Singer et al GCN24069
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 ).

The Fermi Gamma-ray Space Telescope was passing through the South Atlantic
Anomaly (SAA) at the time of the trigger (T0 = 2019-04-08 18:18:02.288 UTC).
During SAA passages the LAT does not collect data due to the high
charged particle background in this region.
The LAT resumed data taking upon exiting the SAA at roughly T0 + 1000 s.
At that time the instantaneous coverage of the LIGO map was ~5%, and reached
100% cumulative coverage within ~3 ks. We define "instantaneous coverage"
as the integral over the region of the LIGO probability map that is
within the LAT
field of view at a given time, and "cumulative coverage" as the integral of the
instantaneous coverage over time.

We performed a search for a transient counterpart within the 90% contour of
LIGO map in the time window from T0 to T0 + 10 ks, and no significant
new sources are found.

We also performed a search which adapted the time interval of the analysis to
the exposure of each region of the sky. No significant candidate counterpart
was found.

Energy flux upper bounds between 100 MeV and 100 GeV for this search vary
between 2e-10 and 7e-10 [erg/cm^2/s].

The Fermi-LAT point of contact for this event is
Elisabetta Bissaldi (Elisabetta.Bissaldi@ba.infn.it).

The Fermi-LAT is a pair conversion telescope designed to cover
the energy band from 20 MeV to greater than 300 GeV
It is the product of an international collaboration between
NASA and DOE in the U.S. and many scientific institutions
across France, Italy, Japan and Sweden.