Author: Charles Dermer
Abstract: One of the most interesting GRB discoveries of the Fermi mission is the delayed onset of the 100 MeV - GeV emission following the start of the 100 keV - MeV radiation that triggers the burst detector. The MeV photons are generally thought to be a consequence of colliding shells, and the LAT GeV photons are radiation from an external shock made by a highly relativistic outflow sweeping up circumstellar material. In both cases the radiation mechanism is primarily nonthermal synchrotron. We show that the different νFν peak frequencies at MeV and GeV energies follows from a colliding shell/external shock origin, because the electron Lorentz factor is determined by the relative bulk Lorentz factors formed in the shock, which is of order 5-20 for colliding shells, and of order 1000 for the external shock. The bulk Lorentz factors and magnetic field in the radiating fluid are derived from an equipartition approach recently developed to apply to blazars, from which we independently derive bulk Lorentz factors ∼1000 and magnetic fields of a few G.