GCN Circular 26556

*LIGO/Virgo S191222n: No counterpart candidates in Fermi-LAT observations *

E. Moretti (IFAE, Barcelona), M. Crnogorcevic (Univ. of Maryland &
NASA/GSFC), F. Longo (UniTS and INFN, Trieste) and M. Axelsson (KTH &
Stockholm Univ.) report on behalf of the Fermi-LAT Collaboration:

We have searched data collected by the Fermi Large Area Telescope (LAT) on
Dec 22, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission
in spatial/temporal coincidence with the LIGO/Virgo trigger S191222n (GCN
26543
Not found
 
 
).

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.

The Fermi Gamma-ray Space Telescope was passing through the South
Atlantic Anomaly (SAA) at the time of the trigger (T0
= 2019-12-22 03:35:37.118 UTC). During SAA passages both the LAT and the
Gamma-ray Burst Monitor (GBM) do not collect data due to the high charged
particle background in this region. The LAT resumed taking data upon
exiting the SAA at roughly T0 + 1ks. At that time the instantaneous
coverage was 55% of the LIGO probability map, and reached cumulative
coverage of 90% after ~5 ks. The remaining area was not observed within 10
ks following the trigger time of the event.

We performed a search for a transient counterpart within the observed
region of the 90% contour of LIGO map in a fixed time window from T0 + 1ks
to T0 + 10 ks. 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, and no additional excesses were
found.

Energy flux upper bounds for the fixed time interval between 100 MeV and 1
GeV for this search vary between 1.4e-10 and 5.7e-08 [erg/cm^2/s].

The Fermi-LAT point of contact for this event is Milena Crnogorcevic (
mcrnogor@astro.umd.edu).

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.

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