THE HIGH-ENERGY TAIL OF GRB 941017: COMPTONIZATION OF SYNCHROTRON SELF-ABSORBED PHOTONS

摘要

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 approx> 5 x 10~(53) ergs (assuming spherical symmetry); an upper limit to the density of gas surrounding the explosion, n approx< 10~(-2)(E/10~(54) ergs) cm~(-3); a lower limit to the expansion Lorentz factor Γ_i approx> 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 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.