TP-31

Beaming and Baryon-Loading Effects in Gamma-Ray Burst Blast Waves Energized by External Shocks

Charles D. Dermer (NRL), Kurt E. Mitman (NRL, TJHSST), James Chiang (JILA)

We present detailed calculations of nonthermal synchrotron and synchrotron self-Compton (SSC) spectra produced 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. Because no lateral spreading of the blast-wave is assumed, the calculated temporal breaks represent the sharpest possible breaks from collimated outflows in a uniform surrounding medium, thus providing a test for GRB beaming. Observations of the time dependence of the 100 MeV-TeV spectral indices to chart the evolution of the SSC component in the early afterglow phase can discriminate between the external shock scenario and a scenario involving colliding shells. 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 unassociated with GRBs will not unambiguously demonstrate GRB beaming.