TITLE:   GCN CIRCULAR
NUMBER:  30943
SUBJECT: GRB 211018A: Fermi-LAT detection
DATE:    21/10/19 08:34:58 GMT
FROM:    Magnus Axelsson at Stockholm U.  <magaxe@kth.se>

M. Arimoto (Kanazawa Univ.), M. Axelsson (KTH & Stockholm Univ.), N. Di Lalla (Stanford Univ.), N. Omodei (Stanford Univ.), F. Longo (Univ. and INFN, Trieste) and D. Tak (DESY) report on behalf of the Fermi-LAT Collaboration:

On October 18th, 2021, Fermi-LAT detected high-energy emission from GRB 211018A, which was also detected by Fermi GBM (trigger 656288898 / 211018936). The best LAT on-ground location is found to be

RA, Dec = 19.68, -3.60 (degrees, J2000)

with an error radius of 0.19 deg (90% containment, statistical error only).

The position was 68 deg from the LAT boresight at the time of the GBM trigger:

T0 = 22:28:13.9 UT.

The data from the Fermi-LAT show a significant increase in the event rate after the trigger that is temporally and spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0-300 s after the trigger is 1.2 (-/+ 0.3) e-04 ph/cm2/s.

The estimated photon index above 100 MeV is -2.0 (-/+ 0.2).

The highest-energy photon is a 3.3 GeV event which is observed 25 seconds after the GBM trigger.

A Swift ToO has been requested for this burst.

The Fermi-LAT point of contact for this burst is Donggeun Tak (takdg123@gmail.com).

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.