Dynamical and Radiative Properties of X-Ray Pulsar Accretion Columns: Effects of Gas and Radiation Pressure




West, Brent Frederick

Journal Title

Journal ISSN

Volume Title



Previous research to investigate the dynamics of luminous X-ray pulsars and the observed spectra has largely been confined to the single-fluid model in which the higher luminosity permits the accreting flow to be regarded as a radiation-dominated ideal fluid. In this regime, the inflowing ionized gas held no special significance when investigating the dynamics of accretion column formation and the associated radiation-dominated standing shock through which the fluid must pass. This PhD research examines the dynamical importance of gas pressure in both low-luminosity and high-luminosity X-ray pulsars in which the pressure of the ionized gas may play a significant role in column formation and its associated dynamics. The “two-fluid” model is implemented by coupling radiation and gas as interacting fluids. The fluids pass through a radiation sonic point located in a shock wave where the radiation sound speed equals the bulk fluid speed. The precise location of the sonic point largely depends upon the details of the upstream boundary conditions for the incident radiation and gas sound speeds. The parameter space for the incident sound speeds is mapped and the associated temperature, pressure, and density distributions are calculated as functions of the altitude in the column. The complete dynamical problem is fully modeled by defining five fundamental free parameters, namely: (1) the polar cap size, (2) the altitude at the top of the accretion column, (3) the incident radiation Mach number, (4) the parallel scattering cross-section, and (5) the angle-averaged scattering cross-section. All of the other model parameters are derived from these fundamental free parameters. The resulting X-ray spectral formation is investigated through numerical computation based on the transport equation developed by Becker & Wolff (2007) which accounts for the bulk and thermal Comptonization inside the accreting gas. The Becker & Wolff (2007) model generally gives good agreement with the observational data for high-luminosity pulsars. However, that model did not include a self-consistent hydrodynamical calculation of the velocity profile for the accreting gas. This PhD research extends the Becker & Wolff (2007) model by self-consistently calculating the velocity profile in a conical geometry, including the dynamical effect of both the gas pressure and the radiation pressure. The resulting X-ray spectra are compared with the observations for a variety of sources covering a wide range of luminosity. The resulting parameter values are compared with those obtained using the Becker & Wolff (2007) model. Consideration of the energy and angular dependencies of the electron scattering cross section will allow a more detailed interpretation of the observed energy-dependent pulse profiles, allowing us to obtain a deeper understanding of the extreme physics occurring in these sources.



X-Ray pulsar, Radiation-Dominated pulsar, Accretion column, Accretion of material, Neutron Star, Low-Luminosity pulsar