ACCRETION POWERED PULSARS - CONTINUUM SPECTRA AND LIGHT CURVES OF SETTLING ACCRETION MOUNDS

We apply the radiative transfer technique described in Burnard, Klein, and Arons to model the polarimetric spectra expected from a monud of accreting plasma settling subsonically onto the polar cap regions of an X-ray pulsar. We employ an analytical model of the accretion flow near the polar caps of the neutron stars, described briefly here, and demonstrate how the emission from such an accretion mound can be computed from a series of one-dimensional slabs of nonuniform material. We compare the results of our detailed calculations for these slabs with those obtained from a simple mode-coupled modified blackbody model. We find that the geometry of the elevated mound forces the observer to receive photons from patches whose surfaces subtend a wide range of angles to the line of sight. The resulting averaging of intensity over the surface of the mound greatly reduces the variation in intensity and polarization found in single-slab models. We conclude that single-slab models are inadequate for representing the emission region of all but the lowest luminosity X-ray pulsars; a self-consistent model of both the accretion flow and the radiation field is required. For the simple mound studied here, in which the mass flux far above the stellar surface is independent of distance from the magnetic axis, the emission from the postshock settling zone does not contain enough low-frequency photons to model the observed spectra of X-ray pulsars, specifically Her X-1. We point out that mound models with steeper gradients and/or additional soft-photon sources are required to explain the large number of low-frequency photons seen in X-ray pulsars. A discussion of the importance of the radiation-dominated shock and its outer sheath (which are neglected in this paper), lead us to suggest that past success achieved using thin uniform backlit slabs can be interpreted dynamically as the Comptonizing layer formed by the shock itself, backlit by the saturated spectrum radiated from mounds of the type described here.