The Inverse Compton Emission Spectra in the Very Early Afterglows of Gamma‐Ray Bursts

We calculate the spectra of inverse Compton (IC) emissions in gamma-ray burst(GRB) shocks produced when relativistic ejecta encounters the externalinterstellar medium, assuming a broken power-law approximation to thesynchrotron seed spectrum. Four IC processes, including the synchrotronself-Compton (SSC) processes in GRB forward and reverse shocks, and twocombined-IC processes (i.e. scattering of reverse shock photons on theelectrons in forward shocks and forward shock photons on the electrons inreverse shocks), are considered. We find that the SSC emission from reverseshocks dominates over other emission processes in energy bands from tens of MeVto tens of GeV, for a wide range of shock parameters. This mechanism may beresponsible for the prompt high energy gamma-rays detected by the EnergeticGamma Ray Experiment Telescope (EGRET). At TeV energy bands, however, thecombined-IC emissions and/or the SSC emission from the forward shocks becomeincreasingly dominant for a moderately steep distribution of shocked electrons.Comment: 15 pages, 4 EPS figures, Latex, accepted for publication in ApJ, scheduled for the v556 n2 Aug 1, 2001 issu