SPECTRAL PROPERTIES OF BLAST-WAVE MODELS OF GAMMA-RAY BURST SOURCES

We calculate the spectrum of blast-wave models of gamma-ray burst sour ces, for various assumptions about the magnetic field density and the relativistic particle acceleration efficiency. For a range of physical ly 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 e nergies. We discuss the spectra as a function of duration for three ba sic types of models, and for cosmological, halo, and galactic disk dis tances. We also evaluate the gamma-ray fluences and the spectral chara cteristics for a range of external densities. Impulsive burst models a t cosmological distances can satisfy the conventional X-ray paucity co nstraint S(x)/S(y) less than a few percent over a wide range of durati ons, but galactic models can do so only for bursts shorter than a few seconds, unless additional assumptions are made. The emissivity is gen erally larger for bursts in a denser external environment, with the ef ficiency increasing up to the point where all the energy input is radi ated away.