2004ApJ...603L...1P

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≳5x10⁵³ ergs (assuming spherical symmetry); an upper limit to the density of gas surrounding the explosion, n≲10^–2(E/10⁵⁴ ergs)/cm3; 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, ε_B,f≤10^–4. Such constraints cannot be inferred from keV-MeV data alone. The unusually low value of ε_B,fand 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.