| Accretion powered X-ray pulsars are investigated in detail. Models of emission regions are developed based on the self-consistent solution of the radiative transfer together with the hydrostatic and radiative equilibrium equations. The dominant opacity is due to electron scattering in a strong magnetic field, and free-free absorption is included, the cross sections being calculated with first order relativistic corrections. Radiation pressure effects are taken into account in detail. Estimates of the critical luminosity in the magnetic case are found to be lower than the gray Eddington luminosity. The dependence of the outgoing spectrum on the energy deposition mechanism is discussed.; X-ray pulsar light curves are simulated taking into account the effects of gravitational bending, general geometry, and a parametrized accretion cap structure. The model spectra as a function of time are compared with observations of two X-ray pulsars, 4U1538-52 and Vela X-1, and satisfactory fits are obtained. The fits suggest some anomalies in the geometry of the objects, e.g. the magnetic axes seem to be off-center, and a quite complex magnetic field structure on the caps is indicated. The accretion cap sizes are found to differ for a given pulsar, being generally compatible with theoretical models of accretion. General relativistic effects are found to be important in the formation of the spectra. The fluxes found in the fits indicate that the effects of radiation pressure cannot be neglected. The work presented here is the first analysis of X-ray pulsar spectra as a function of phase based on a detailed physical model. |
