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Spectral Properties of Blast Wave Models of Gamma-Ray Burst Sources

We calculate the spectrum of blast wave models of gamma-ray burst sources, for various assumptions about the magnetic field density and the relativistic particle acceleration efficiency. For a range of physically plausible models we find that the radiation efficiency is high, and leads to nonthermal spectra with breaks at various energies comparable to those observed in the gamma-ray range. Radiation is also predicted at other wavebands, in particular at X-ray, optical/UV and GeV/TeV energies. We discuss the spectra as a function of duration for three basic types of models, and for cosmological, halo and galactic disk distances. We also evaluate the gamma-ray fluences and the spectral characteristics for a range of external densities. Impulsive burst models at cosmological distances can satisfy the conventional X-ray paucity constraint $S_x/S_γ\siml$ few percent over a wide range of durations, but galactic models can do so only for bursts shorter than a few seconds, unless additional assumptions are made. The emissivity is generally larger for bursts in a denser external environment, with the efficiency increasing up to the point where all the energy input is radiated away.