GCN Circular 46085
F. Longo (Univ. and INFN, Trieste), T. Nymark (KTH), A. Holzmann Airasca (UniTrento and INFN Bari), and R. Gupta (NASA/GSFC) report on behalf of the Fermi-LAT collaboration:
At 01:27:03 UT on October 29th, 2026, Fermi-LAT detected high-energy emission from GRB 261029A, which was also detected by Fermi-GBM (trigger 814930028/261029060, GCN #46071) and SVOM (GCN #46072). Its optical counterpart was found by GOTO (GCN #46073) at a redshift of z = 3.27 using the NOT (GCN #46081).
The best LAT on-ground location is found to be
RA, Dec = 9.69, 35.52 (J2000)
with an error radius of 0.57 deg (90 % containment, statistical error only).
This was 67 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 - 120 s after the GBM trigger is (9.4 ± 1.1)E-4 ph/cm2/s. The estimated photon index above 100 MeV is -2.80 ± 0.17. The highest energy photon has an energy of 2.5 GeV and occurs at 44 s after trigger time.
The Fermi-LAT point of contact for this burst is Riccardo Martinelli (riccardo.martinelli@phd.units.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.