TITLE: GCN CIRCULAR NUMBER: 39010 SUBJECT: GRB 250121A: Fermi-LAT detection DATE: 25/01/22 10:27:34 GMT FROM: A. Holzmann Airasca at University of Trento and INFN Bari a.holzmannairasca@unitn.it

A. Holzmann Airasca (UniTrento and INFN Bari), D. Depalo (Politecnico and INFN Bari), E. Bissaldi (Politecnico and INFN Bari), R. Gupta (NASA GSFC), C. Bartolini (UniTrento and INFN Bari), S. Lopez (CNRS / IN2P3) on behalf of the Fermi-LAT Collaboration:

On January 21, 2025, Fermi-LAT detected high-energy emission from GRB 250121A, which was also detected by Fermi-GBM (trigger 759195370 / 250121983, GCN 39006).

The best LAT on-ground location is found to be: RA, Dec = 225.14, -34.37 (J2000)

with an error radius of 0.24 deg (90 % containment, statistical error only). This was 15 deg from the LAT boresight at the time of the GBM trigger (T0 = 23:36:05.78 UT). The data from the Fermi-LAT shows a significant increase in the event rate that is spatially and temporally correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0 - 500 s after the GBM trigger is (2.5 ± 1.2) E-6 ph/cm2/s. The estimated photon index above 100 MeV is 1.7 ± 0.3. The highest-energy photon is a 2.6 GeV event which is observed ~ 15 seconds after the GBM trigger.

The Fermi-LAT point of contact for this burst is Aldana Holzmann Airasca (aldana.holzmannairasca@ba.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.

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