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The extended high-energy gamma-ray (>100 MeV) emission which occurs wellafter the prompt gamma-ray bursts (GRBs) is usually explained as the afterglowsynchrotron radiation. Here we report the analysis of the Fermi Large AreaTelescope observations of GRB 130907A. A 55 GeV photon compatible with theposition of the burst was found at about 5 hours after the prompt phase. Theprobability that this photon is associated with GRB 130907A is higher than99.96%. The energy of this photon exceeds the maximum synchrotron photon energyat this time and its occurrence thus challenges the synchrotron mechanism asthe origin for the extended high-energy >10 GeV emission. Modeling of thebroad-band spectral energy distribution suggests that such high energy photonscan be produced by the synchrotron self-Compton emission of the afterglow.

An inverse Compton origin for the 55 GeV photon in the late afterglow of GRB 130907A