The high energy tail of gamma-ray burst 941017: Comptonization of synchrotron self absorbed photons

The recent detection of an unusually hard spectral component in GRB941017 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 emission 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>~ 5x10^53 erg (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} <= 10^{-4}. Such constraints can not be inferred from keV--MeV data alone. The unusually low value of \epsilon_{B,f} may account for the rareness of GRB941017-type high energy tails. Tighter constraints on model parameters may be obtained in the future from optical and sub-TeV observations