TITLE: GCN CIRCULAR NUMBER: 43034 SUBJECT: GRB 251208B: Fermi-LAT detection DATE: 25/12/08 18:51:48 GMT FROM: chiara.bartolini-1@unitn.it
E. Bissaldi (Politecnico and INFN Bari), N. Di Lalla (Stanford University), R. Gupta (NASA/GSFC), A. Holzmann Airasca (UniTrento and INFN Bari), C. Bartolini (UniTrento and INFN Bari) and F. Longo (University and INFN, Trieste) report on behalf of the Fermi-LAT team:
At 10:17:17.29 UT on December, 08, 2025 Fermi-LAT detected high-energy emission from GRB 251208B, which was also detected by Fermi-GBM (trigger 786881842 / 251208429, GCN #43028) and Swift/BAT-GUANO (GCN #43033).
The best LAT on-ground location is found to be
RA, Dec = 228.48, 29.07 (J2000)
with an error radius of 0.07 deg (90 % containment, statistical error only), which is consistent with the Swift/BAT-GUANO localization.
This was 39 deg from the LAT boresight at the time of the trigger.
The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the trigger with high significance. The photon flux above 100 MeV in the time interval 0 - 1 ks after the GBM trigger is (3.13 ± 0.09)E-6 ph/cm2/s. The estimated photon index above 100 MeV is -2.01 ± 0.23. The highest-energy photon is a 17 GeV event which is observed 419 seconds after the GBM trigger.
A Swift ToO has been approved for this burst.
The Fermi-LAT point of contact for this burst is Chiara Bartolini (chiara.bartolini@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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