Beaming, baryon loading, and the synchrotron self-compton component in gamma-ray bursts

We present detailed calculations of nonthermal synchrotron and synchrotron self-Compton (SSC) spectra radiated by blast waves that are energized by interactions with a uniform surrounding medium. Radio, optical, X-ray, and gamma-ray light curves and spectral indices are calculated for a standard parameter set that yields hard GRB spectra during the prompt emission phase. No lateral spreading of the blase wave is assumed. Absence of SSC hardenings in observed GRB X-ray afterglows indicates magnetic field generation toward equipartition as the blast wave evolves. EGRET detections of 100 MeV-GeV photons observed promptly and 90 minutes after GRB 940217 are attributed to nonthermal synchrotron radiation and SSC emission from a decelerating blast wave, respectively. The SSC process will produce prompt TeV emission that could be observed from GRBs with redshifts z less than or similar to 0.1, provided gamma-gamma opacity in the source is small. Measurements of the time dependence of the 100 MeV-GeV spectral indices with the planned Gamma-ray Large Area Space Telescope mission will chart the evolution of the SSC component and test the external shock scenario. Transient optical and X-ray emissions from misaligned GRBs are generally much weaker than on-axis emissions produced by dirty and clean fireballs that would themselves not trigger a GRB detector; thus, detection of long-wavelength transients not associated with GRBs will not unambiguously demonstrate GRB beaming.