Radiative Transport and Hydrodynamics of Accretion-Powered X-ray Pulsars

Abstract

The problem of spectral formation in accretion-powered X-ray pulsars was solved for the first time using an analytical model in 2007. Based on fundamental physics, the resulting model spectra were shown to agree closely with those observed from several of the most luminous X-ray pulsars. However, to derive the analytical solutions, simplifying assumptions were made regarding the inflow velocity profile, the thermal structure of the plasma, the boundary conditions, and the geometry of the column. In this dissertation, the problem is revisited using a new numerical approach that facilitates the solution of a more realistic, coupled radiative-hydrodynamic model. The new model utilizes a conical geometry for the accretion flow and applies a robust free-streaming boundary condition at the top of the column. Because of the extreme matter density just above the surface of the neutron star, photons cannot penetrate to the stellar surface. The model imposes a “mirror” boundary condition at the neutron star surface which allows no radiation flux into the star. The temperature of the electrons is computed based on inverse-Compton equilibration instead of computing it from the plasma’s equation of state. The hydrodynamic structure of the column is determined by solving the coupled set of conservation equations for mass, momentum, and energy. The column-integrated spectrum computed using the new selfconsistent, radiative hydrodynamic model agrees well with the data for the Hercules X-1 pulsar. The physical significance of the results will be discussed and a comparison of the resulting source parameters will be made with those computed using the original analytical model.