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.
