TITLE: GCN CIRCULAR NUMBER: 35416 SUBJECT: GRB 231222B: Fermi-LAT detection DATE: 23/12/22 21:31:22 GMT FROM: N. Di Lalla at Stanford University niccolo.dilalla@stanford.edu

N. Di Lalla (Stanford University), F. Longo (University and INFN, Trieste), A. Holzmann (DF, University of Buenos Aires, Argentina) report on behalf of the Fermi-LAT Collaboration:

On December 22, 2023 Fermi-LAT detected high-energy emission from GRB 231222B, which was also detected by Fermi-GBM (trigger 724922767 / 231222310).

The best LAT on-ground location is found to be

RA, Dec = 251.35, 19.55 (degrees, J2000)

with an error radius of 0.3 deg (90% containment, statistical error only). This was 19 deg from the LAT boresight at the time of the GBM trigger:

T0 = 07:26:02.19 UT.

The data from the Fermi-LAT show a significant increase in the event rate after the GBM trigger that is spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval**0-600 s after the GBM trigger is (1.9 +/- 0.7) E-6**ph/cm2/s.

The estimated photon index above 100 MeV is -1.7 +/- 0.3. The highest-energy photon is a 3.6 GeV event which is observed 362 seconds after the GBM trigger.

A Swift ToO has been approved for this burst.

The Fermi-LAT point of contact for this burst is Makoto Arimoto (arimoto@se.kanazawa-u.ac.jp mailto:arimoto@se.kanazawa-u.ac.jp)

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.

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