Steady mildly relativistic thermal plasmas: processes and properties

Abstract

The physical properties of a finite, thermal plasma in pair balance are investigated as a function of the three dimensionless parameters: (1) the temperature θ, (2) the "proton optical depth" τp, and (3) the proton density np. At a dimensionless luminosity l ⪆ 1 Comptonization of bremsstrahlung and double Compton photons dominates the cooling. A determination of the steady pair density in such a Wien equilibrium plasma shows the existence of the two pair density branches previously found for optically thin plasmas. The complicated interplay between photon and pair processes in such a plasma is elucidated. The main result is the existence of a pair-dominated luminosity curve that separates a pair-free region from a forbidden one in l - θ space. Requiring pair balance limits allowed luminosities at mildly relativistic temperatures to values two orders of magnitude smaller than the Eddington luminosity normally employed in accretion models.