GAMMA-GAMMA ATTENUATION AND RELATIVISTIC BEAMING IN GAMMA-RAY BURSTS

The observation by the BATSE experiment on board the Compton Gamma-Ray Observatory (CGRO) that the gamma-ray burst population is isotropic but inhomogeneous has dramatically bolstered the premise that most bursts are cosmological. At the same time, BATSE has also observed definite spectral breaks (Schaefer et al (1992) between 400 keV and 2 MeV in some bursts, which in three cases have been corroborated by data from EGRET. The presence of these breaks in these sources suggests attenuation by the pair production mechanism gammagamma --> e+e-, which can absorb gamma rays, though it is suppressed when the radiation is highly beamed. In this paper, quantitative constraints that pair production places on radiation collimation in individual sources, due to the presence or absence of breaks in spectra, are presented. Cosmological bursts have extremely relativistic beaming, though the bulk Lorentz factors of their emission regions are reduced markedly for steep source spectra. Such beaming will blueshift the breaks to higher energies, indicating that those CGRO bursts displaying MeV spectral breaks may not be cosmological, or perhaps that their breaks are generated by mechanisms that dominate gamma-gamma attenuation at MeV energies. Alternative mechanisms for break production are discussed.