Relativistic thermal plasmas - Radiation mechanisms

We investigate several important radiation mechanisms in relativistic thermal plasmas, obtaining approximate analytic expressions rather than very detailed, numerical results. In particular, we obtain approximate spectra for pair annihilation radiation and double Compton radiation. We also calculate spectra for Comptonized bremsstrahlung radiation as a function of temperature and scattering depth of the medium, including the factor by which Comptonization amplifies the total photon energy. We discuss the conditions under which various of these processes dominate and make some comparisons to the nonrelativistic regime. As in the nonrelativistic case, double Compton is only important for radiation dominated plasmas, and then only for kT/mc/sup 2/< or approx. =5. Annihilation radiation is only important at values of Thomson scattering depth tau/sub th/ < or approx. =0.3, and kT/mc< or approx. =10. At higher values of tau/sub th/, inverse Compton scattered bremsstrahlung photons dominate at the annihilation energy peak, hv/kTapprox.2. At higher temperatures, the un-Comptonized bremmstrahlung photons dominate everywhere. These radiation processes will be important for determining the electron-positron pair densities in relativistic plasmas, as well as for the emergent spectra from such plasmas.