GCN Circular 41884
Subject
GRB 250919A: Fermi-LAT detection
Event
Date
2025-09-19T09:21:09Z (25 days ago)
From
A. Holzmann Airasca at University of Trento and INFN Bari <a.holzmannairasca@unitn.it>
Via
Web form
A. Holzmann Airasca (UniTrento and INFN Bari), N. Di Lalla (Stanford Univ.), E. Bissaldi (Politecnico and INFN Bari), S. Lopez (CNRS / IN2P3), D. Depalo (Politecnico and INFN Bari), T. Khalil (Johannesburg Univ), R. Gupta (NASA/GSFC) and F. Longo (University and INFN, Trieste) report on behalf of the Fermi-LAT Collaboration:
On September 19, 2025, Fermi-LAT detected high-energy emission from GRB 250919A, which was also detected by Fermi-GBM (trigger 779934537 / 250919020, GCN 41876), EP (GCN 41879) spatially coincident but temporally offset by 1.77 hours, SVOM/GRB (GCN 41882) and Gemini/GMOS-S (GCN 41883).
The best LAT on-ground location is found to be:
RA, Dec = 298.52, -48.83 (J2000)
with an error radius of 0.08 deg (90 % containment, statistical error only).
This was 56 deg from the LAT boresight at the time of the GBM trigger (T0 = 00:28:52.28 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 - 1.7 ks after the GBM trigger is (1.03 ± 0.14) E-5 ph/cm2/s. The estimated photon index above 100 MeV is -1.84 ± 0.09.
The highest-energy photon is a 7 GeV event which is observed ~ 980 seconds after the GBM trigger.
The Fermi-LAT point of contact for this burst is Oscar Wistemar (wistemar@kth.se).
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.