GAMMA-RAY BURST SPECTRAL BREAKS AND SOURCE BEAMING

The principal discovery of the Compton Gamma-Ray Observatory (GRO) con cerning gamma-ray bursts is that these sources are isotropic but with a comparative deficiency of fainter sources, suggesting that they are probably cosmological in origin. If they are at such large distances f rom Earth then they are extremely luminous and compact. A consequence of this is that two-photon pair production attenuation of the gamma-ra y continuum cannot be avoided unless the source radiation is substanti ally beamed. Most sources do not display gamma-ray turnovers although a few GRB detected by GRO exhibit distinct spectral breaks in the MeV range. A derivation of the relationship defining of the degree of beam ing in burst sources with spectral breaks due to gamma-gamma attenuati on, as a function of source spectral index and break energy, is presen ted. It is found that sources at distances of similar to 1 Gpc must ty pically be beamed with bulk Lorentz factors of around 10(3)-10(4), ind icating powerful bulk acceleration in bursts, although these Lorentz f actors are reduced markedly for steep source spectra. Since the source spectra are not strongly Comptonized, such beaming will blueshift the gamma-gamma attenuation breaks to energies much higher than 1 MeV; an absolute lower bound to the source bulk Lorentz factor is determined from this additional constraint. This blueshifting suggests that those sources with MeV breaks may not be cosmological, or that their breaks are produced by a mechanism that dominates gamma-gamma attenuation at these energies.