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

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 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 ≳ 5 × 10 53 ergs (assuming spherical symmetry); an upper limit to the density of gas surrounding the explosion, n ≲ 10 -2 ( E /10 54 ergs) cm -3 ; a lower limit to the expansion Lorentz factor Γ i ≳ 200; and an upper limit to the fraction of thermal energy carried by the magnetic field behind the shock driven into the surrounding medium, B , f ≤ 10 -4 . Such constraints cannot be inferred from keV-MeV data alone. The unusually low value of B , f and the unusually high ratio E / n may account for the rareness of GRB 941017-type high-energy tails. Tighter constraints on model parameters may be obtained in the future from optical and sub-TeV observations.