TITLE:   GCN CIRCULAR
NUMBER:  20777
SUBJECT: LIGO/Virgo G275697: Fermi-LAT search for a high-energy gamma-ray counterpart
DATE:    17/02/28 23:05:48 GMT
FROM:    Giacomo Vianello at Stanford U/Fermi LAT  <giacomo.slac@gmail.com>

G. Vianello (Stanford), N.Omodei (Stanford), Daniel Kocevski (NASA/MSFC)
and Sara Buson (GSFC) report on behalf of the Fermi-LAT team:


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 G275697.

Fermi-LAT had an instantaneous coverage of ~20% at the time of the trigger
(T0 =  2017-02-27 18:57:31.375 UTC), and reached 100% cumulative coverage
within ~7 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 the
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  3e-10 – 3e-9 [erg/cm^2/s].

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.