Comptonization in strongly magnetized and nonmagnetized plasmas

We present a unified treatment of the continuum radiative transfer in optically thick hot plasmas where incoherent Compton scattering is important, either in the presence or absence of strong magnetic fields, as occurs in X-ray emitting neutron stars. Numerical solutions of the transfer equations are obtained and compared to a simplified analytical treatment, including the effect of stimulated scattering in an approximate manner. The latter effect is essential in order to obtain the correct spectrum at frequencies ω ≪ kT. In field-free plasmas the Comptonization drastically changes the outgoing spectrum if ( N_e 10²² cm^-3) ^{1 2}(kT/10 keV)^{- 9 4} ≪ 10, where N_e and T are the electron density and temperature of the plasma. In strongly magnetized plasmas (ω_B≫kT, where ω_B is the cyclotron frequency) the Comptonization effects are less significant than the magnetic effects at frequencies ω ≪ ω_B, for parameters typical of accreting pulsars. The Comptonization is significant only for the ordinary polarization mode, which is much less intense than the extraordinary polarization mode. On the contrary, the Comptonization changes essentially all the properties of the radiation escaping at frequencies close to the cyclotron resonance, ω {reversed tilde equals} ω_B. These results are useful in the optically thick regions of neutron star models of gamma ray bursters, and are of major relevance for an appropriate treatment of models of accreting pulsars and X-ray bursters.