Gamma-ray Bursts: Multiwaveband Spectral Predictions for Blast Wave Models

In almost any scenario for ’cosmological’ gamma-ray bursts (and in many models where they originate in our own Galaxy), the initial energy density is so large that the resulting relativistic plasma expands with v ∼ c producing a blast wave ahead of it and a reverse shock moving into the ejecta, as it ploughs into the external medium. We evaluate the radiation expected from these shocks, for both cosmological and galactic bursts, for various assumptions about the strength of the magnetic field and the particle acceleration mechanisms in the shocks. The spectra are evaluated over the whole range from the IR to > GeV, and are compared with the variety of spectral behavior reported by BATSE, and with the X-ray and optical constraints. For bursts of duration >∼ 1 s acceptable γ-ray spectra and Lx/Lγ ratios are readily obtained for ’cosmological’ models. Blast waves in galactic models can produce bursts of similar gamma-ray fluence and duration, but they violate the X-ray paucity constraint, except for the shorter bursts (<∼ 1 s). We discuss the prospects for using O/UV and X-ray observations to discriminate among alternative models.