GRB 210928A: Fermi-LAT detection

On September 28th, 2021, Fermi-LAT detected high-energy emission from GRB 210928A, which was also detected by Fermi GBM (trigger 654487236 / 210928084). This source has previously been reported as the Fermi-LAT transient J1623-1752 (Rani et al., ATel #14939) and Swift X-ray transient J162315.0-175233 (Page et al.; ATel #14945). Further analysis shows that the LAT source is consistent with being a GRB, making the Swift transient the afterglow candidate. The best LAT on-ground location is found to be RA, Dec = 245.88, -17.91 (degrees, J2000) with an error radius of 0.15 deg (90% containment, statistical error only). This location is consistent with the Swift X-ray transient. The position was 16 deg from the LAT boresight at the time of the GBM trigger: T0 = 02:00:31 UT. The data from the Fermi-LAT show a significant increase in the event rate after the trigger that is spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0-1000 s after the trigger is 8.4 (-/+ 1.5) e-06 ph/cm2/s. The estimated photon index above 100 MeV is -2.2 (-/+ 0.2). The highest-energy photon is a 6 GeV event which is observed ~50 s after the trigger. The Fermi-LAT point of contact for this burst is Magnus Axelsson (magaxe@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.