Author: Andrei M. Beloborodov
Abstract: GeV emission in GRBs shows a puzzling temporal behavior. It starts with a slight delay, sharply peaks near the beginning of the MeV burst, and lasts much longer than the MeV emission. We discuss possible mechanisms of GeV emission and identify one that appears to be consistent with data: inverse Compton scattering of the prompt MeV burst in the external blast wave. Through radiative transfer simulations of this model, we find that it explains observations and opens a new window on the nature of circum-burst medium and the explosion Lorentz factor. We discuss implications of our results for GRB progenitors, physics of the prompt burst and its afterglow emission.