Radiation gasdynamics of polar CAP accretion onto magnetized neutron stars - Basic theory

Abstract

The dynamics of plasma flow in the low-altitude regions of high-luminosity accreting, magnetized neutron stars is studied. Useful expressions for bremsstrahlung emission and absorption in a strong magnetic field are derived along with expressions for the collisional creation of cyclotron line photons. Approximations for the resonant and nonresonant parts of the scattering cross section are discussed, and a useful expression for the rate of Compton energy exchange is derived. Radiation transfer is formulated, arguing that birefringence has negligible effects on the overall dynamical problem. Angle- and frequency-averaged equations for the flow of radiative energy and number density in the diffusion approximation are derived, including the contribution to the radiative energy flux made by the advection of radiative enthalpy. A new 'on-the-spot' approximation for the transfer of cyclotron line photons is developed and used to produce final conservation equations for number, momentum, and energy densities of photons. The conservation equations for the plasma are presented and used to analyze typical rates for various processes, which in turn are used to estimate the relative magnitudes of the electron and photon temperatures.