E. Bissaldi (Politecnico & INFN Bari), M. Axelsson (KTH ans Stockholm Univ.),
G. Vianello (Stanford Univ.), F. Longo (Univ. & INFN Trieste), and
D. Kocevski (NASA/MSFC) report on behalf of the Fermi-LAT team:
On November 20, 2018, Fermi-LAT detected high-energy emission from GRB 181120A,
which was also detected by Fermi-GBM (trigger 181120265 / 564387684).
The best LAT on-ground location is found to be
RA, Dec: 20.38, 4.17 (degrees, J2000)
with an error radius of 1 deg (90% containment, statistical error only).
This was 71 deg from the LAT boresight at the time
of the GBM trigger: T0 = 06:21:19.08 UT.
The data from the Fermi-LAT show a significant (~5 sigma) increase
in the event rate in the time interval 3000-6000 s after the
GBM emission. We note that the LAT location is 16 degrees from the GBM one.
Based on our analysis, we rule out contamination from the Earth limb,
and the nearest known point source is ~2 degrees away.
The photon flux above 100 MeV in the time interval 3000-6000 s after the
GBM trigger is (2.2 +/- 0.7)E-06 ph/cm2/s.
The estimated photon index above 100 MeV is 4.0 +/- 0.9.
We note that the spectrum is softer than the one typically seen from GRBs.
The highest-energy photon is a 410 MeV event which is
observed 5530 seconds after the GBM trigger.
A Swift ToO has been requested for this burst.
The Fermi-LAT point of contact for this burst is
Elisabetta Bissaldi (firstname.lastname@example.org).
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.