The High-Energy Tail of GRB 941017: Comptonization of Synchrotron Self-absorbed Photons

Abstract

The recent detection of an unusually hard spectral component in gamma-ray burst GRB 941017 extending to >= 200 MeV is hard to explain as a synchrotron emission from shock-accelerated electrons. It was argued to imply acceleration of protons to ultra-high energy. We show here that the "high-energy tail" can be explained as emissoin from shock-accelerated electrons in the early afterglow epoch, taking into account the effect of synchrotron self-absorption. High-energy observations set in this case stringent constraints on model parameters: a lower limit to the total explosion energy E>~ 5 x 10^53 ergs (assuming spherical symmetry); an upper limit to the density of gas surrounding the explosion, n ~ 200; and an upper limit to the fraction of thermal energy carried by the magnetic field behind the shock driven into the surrounding medium, epsilon(B,f)