Spectral Formation in X-Ray Pulsar Accretion Columns

We present the first self-consistent model for the dynamics and the radiativetransfer occurring in bright X-ray pulsar accretion columns, with a specialfocus on the role of the shock in energizing the emerging X-rays. The pressureinside the accretion column of a luminous X-ray pulsar is dominated by thephotons, and consequently the equations describing the coupledradiative-dynamical structure must be solved simultaneously. Spectral formationin these sources is therefore a complex, nonlinear phenomenon. We obtain theanalytical solution for the Green's function describing the upscattering ofmonochromatic radiation injected into the column from the thermal mound locatednear the base of the flow. The Green's function is convolved with a Planckdistribution to model the X-ray spectrum resulting from the reprocessing ofblackbody photons produced in the thermal mound. These photons diffuse throughthe infalling gas and eventually escape out the walls of the column, formingthe observed X-ray spectrum. We show that the resulting column-integrated,phase-averaged spectrum has a power-law shape at high energies and a blackbodyshape at low energies, in agreement with the observational data for many X-raypulsars.Comment: Accepted for publication in ApJ Letters. Several typos noticed during the proof review were correcte