TITLE:   GCN CIRCULAR
NUMBER:  21439
SUBJECT: LIGO/Virgo G296853: Fermi-LAT search for high-energy gamma-ray counterpart
DATE:    17/08/09 20:07:38 GMT
FROM:    Giacomo Vianello at Stanford U/Fermi LAT  <giacomo.slac@gmail.com>

F.Longo (University and INFN, Trieste), N.Omodei, G.Vianello
(Stanford), D.Kocevski (NASA/MSFC) and S.Buson (NASA/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 G296853.

At the time of the trigger (T0 =  2017-08-09 08:28:21.747 UTC,
523960106.747 MET), none of the LIGO Bayestar probability map was in
the LAT field of view. Part of the region entered the LAT field of
view 1700 seconds after T0, and we reached 100% cumulative coverage
within ~2.7 ks after the trigger. 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. 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.  Similarly, the Automated Science
Processing search, which looks for variation in flux from known
sources and for new transients on different time scales (Chiang 2012),
did not detect any new transient consistent  within the 90% contour
of the G296853 map, during a six-hour interval from T0-2hr to T0+4hr.

The Fermi-LAT point of contact for this event is Francesco Longo
(francesco.longo@ts.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.

[GCN OPS NOTE(10aug17): Per author's request, the timestamp in
the second paragraph was changed from "02:01:16.492 UT 523960106.748"
to "08:28:21.747 UTC 523960106.747.]