Mean opacities of a strongly magnetized high-temperature plasma

Geometry and dynamical structure of emission regions in accreting pulsars are shaped by the interplay between gravity, radiation, and strong magnetic field, which significantly affects the opacities of a plasma and radiative pressure under such extreme conditions. Quantitative consideration of magnetic plasma opacities is therefore an essential ingredient of any self-consistent modelling of emission region structure of X-ray pulsars (XRPs). We present results of computations of the Rosseland and Planck mean opacities of a strongly magnetized plasma with a simple chemical composition, namely the solar hydrogen/helium mix. We consider all relevant specific opacities of the magnetized plasma including vacuum polarization effect and contribution of electron-positron pairs where the pair number density is computed in the thermodynamic equilibrium approximation. The magnetic Planck mean opacity determines the radiative cooling of an optically thin strongly magnetized plasma. It is by factor of three smaller than non-magnetic Planck opacity at k(B) T < 0.1 E-cyc and increases by a factor of 10(2)-10(4) at k(B) T > 0.3 E-cyc due to cyclotron thermal processes. We propose a simple approximate expression which has sufficient accuracy for the magnetic Planck opacity description. We provide the Rosseland opacity in a tabular form computed in the temperature range 1-300 keV, magnetic field range 3 x 10(10)-10(15) G, and a broad range of plasma densities. We demonstrate that the scattering on the electron-positron pairs increases the Rosseland opacity drastically at temperatures > 50 keV in the case of mass densities typical for accretion channel in XRPs.

Automated summary

The geometry and dynamical structure of emission regions in accreting pulsars are influenced by the interplay between gravity, radiation, and strong magnetic field. This study focuses on the quantitative consideration of magnetic plasma opacities in the modeling of emission regions in X-ray pulsars. The results show that the magnetic Planck mean opacity determines the radiative cooling of a strongly magnetized plasma and its value varies with temperature and magnetic field strength.