TP-30

Spectral Energy Distributions of Gamma Ray Bursts Energized by External Shocks

Charles D. Dermer (NRL), Markus Böttcher (Rice University), and James Chiang (JILA)

Sari, Piran, and Narayan (1998) have presented analytical formulae to describe the emissions from gamma-ray burst blast waves that are energized by sweeping up material from the surrounding medium. We extend these formulae to include synchrotron self-absorption and synchrotron self-Compton processes, and to apply to general radiative regimes. We find that the analytical formulae are not accurate when the magnetic equipartition parameter $\epsilon_B\ll \epsilon_e$, where $\epsilon_e$ is the fraction of swept-up particle energy that is transferred to the electrons. This is due to the importance of Compton losses on the electrons. Nor are the analytical formulae accurate in regimes intermediate to the fully radiative and fully adiabatic regimes, which occurs when $\epsilon_e \mathrel{\copy\gaxbox}0.1$ and $\epsilon_B \mathrel{\copy\gaxbox}0.01$. We compare the accuracy of the resultant formulae with detailed numerical simulations.