GCN Circular 21227

Nicola Omodei (Stanford), Daniel Kocevski (NASA/MSFC), Giacomo Vianello (Stanford), Judith Racusin, Sara Buson, Jeremy S. Perkins and Julie McEnery (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 G288732.
At the time of the trigger (T0 =  2017-06-08 02:01:16.492 UTC, 518580081.492 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 70 seconds after T0, and we reached 100% cumulative coverage within ~6.5 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, and no significant new sources are found above a Test Statistic (TS) of 25. 
On this time scale, the highest significance excess found was at R.A.,Dec.=128.11, 43.39, (J2000) with a localization error of 0.24 degrees (90% c.l.), with a TS of 23, corresponding to a pre-trial significance of 4.8 sigma. Given the number of trials involved, the post trial significance of this excess is estimated to be ~3.5 sigma.  The location of the candidate was occulted by the Earth at the time of the LIGO trigger, and came into the FoV at ~1200 s after T0.
A Swift ToO observation has been requested to initiate follow-up observations at the location of this candidate source and we encourage additional follow-up observations.

The Fermi-LAT point of contact for this event is: nicola.omodei@stanford.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.