2005A&A...439..957W

The different temporal behaviors of the high energy component and the hundreds of keV emission from GRB 941017 suggest that they come from different emission regions. The nearly constant flux of this high energy component is consistent with being produced in the region of the early external shock formed when the ejecta hit the surrounding medium. Here we show that the relatively hard spectrum of this component can be numerically modelled as the synchrotron self-Compton emission from external reverse shock when the ejecta is decelerated by a typical interstellar medium. Constraints on the parameters of the fireball, such as the initial Lorentz factor η, the burst energy, and the magnetic field equipartition fraction are obtained. Very high initial Lorentz factor (η>10³) inferred for this burst may explain the rarity of this kind of high-energy component in gamma-ray bursts (GRBs). Large GeV-TeV flux from the reverse shock is predicted for GRBs with high initial Lorentz factors.