TITLE:   GCN CIRCULAR
NUMBER:  23561
SUBJECT: GRB 181225A: Fermi-LAT Detection
DATE:    18/12/25 21:42:26 GMT
FROM:    Makoto Arimoto at Tokyo Inst of Tech  <arimoto@hp.phys.titech.ac.jp>

M. Arimoto (Kanazawa Univ.), M. Palatiello (University of Udine and
INFN Trieste),
F. Longo (University and INFN, Trieste), D. Kocevski (NASA/MSFC),
E. Bissaldi (Politecnico and INFN, Bari), G. Vianello (Stanford), and
E. Moretti (IFAE) report on behalf of the Fermi-LAT team:

On December 25, 2018, Fermi-LAT detected high-energy emission from GRB 181225A,
which was also detected by Fermi-GBM (trigger 567431055 / 181225489)
and AGILE-MCAL (A. Ursi et al., GCN 23560).
The best LAT on-ground location is found to be

RA, Dec = 348.07, -9.48 (degrees, J2000)

with an error radius of 0.13 deg (90% containment, statistical error only).
This was 44 deg from the LAT boresight at the time of the GBM trigger:

T0 =  11:44:10.5 UT.

The data from the Fermi-LAT show a significant increase
in the event rate after the GBM trigger that is temporally correlated with the
GBM emission with high significance.
The photon flux above 100 MeV in the time interval 0-1000 s after the
GBM trigger is (2.4 +/- 0.5) E-5 ph/cm2/s, the one above 1 GeV is (1.5
+/- 0.5)E-5 ph/cm2/s.

The estimated photon index above 100 MeV is -2.24 +/- 0.18.

The highest-energy photon is a 13 GeV event which is observed ~4 seconds
after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is
Michele Palatiello (michele.palatiello@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.