Powerful GeV emission from a $\sf \gamma$-ray-burst shock wave scattering stellar photons

Abstract

The γ -ray bursts (GRBs) of long duration are very likely to be connected to the death of massive stars. The γ -ray emission is believed to come from energy released internally in a flow that moves at ultrarelativistic speed. The fast flow drives a shock wave into the external medium leading to the afterglow emission. Most massive stars form in dense clusters, their high luminosity producing a very dense radiation field. Here, I explore the observational consequences of the interaction of the shocked external medium of the burst with the photon field of a nearby O star. I show that inverse Compton scattering of the stellar photons by electrons heated by the shock leads to powerful γ -ray emission in the ~1–100 GeV range. This emission appears minutes to hours after the burst and can be easily detected by Cherenkov telescopes and probably with the GLAST satellite. This signal may have already been observed in GRB 940217 and can yield important information about the circumburst environment and the extragalactic background light.