GCN Circular 15357

G. Vianello (Stanford), R. Desiante (University of Udine and INFN Trieste)
and F.Longo (University of Trieste and INFN Trieste) report on behalf of
the Fermi-LAT team:

At 16:08:39 on 2013-10-18 Fermi LAT detected high energy emission from GRB
131018B, which was also detected by Fermi-GBM ( trigger bn131018673).

The best LAT on-ground location is found to be (RA, DEC) = 304.41, 23.11
(J2000) with an error radius of 0.13 deg (68% containment, statistical
error only), which was 25 deg from the LAT boresight at the time of the
trigger. This position is 12 deg away from the best available GBM
localization. This can be explained by the large statistical error on the
GBM position (4 deg) and its typical systematic error, which is of the
order of few degrees.

The data from the Fermi LAT show an increase in the event rate within 10
degrees of the reported position starting few seconds after the GBM trigger
(a delay common to many Fermi-LAT GRBs), which behaves spatially and
temporally like a new transient point source. This excess has a high
significance (~6 sigma).

More than 10 photons above 100 MeV are observed within 2000 seconds after
the trigger. The highest energy photon is a 13 GeV event which is observed
250 seconds after the GBM trigger.

A Swift ToO request for this burst has been submitted.

The Fermi LAT point of contact for this burst is Rachele Desiante (
rachele.desiante@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.