DYNAMIC EFFECTS ON CYCLOTRON SCATTERING IN PULSAR ACCRETION COLUMNS

The bulk infall motion of an X-ray pulsar accretion column alters the spectrum and the directionality of the primary radiation produced at the base of the polar cap. We examine this effect in the regime where the accretion column is optically thin to Thomson scattering and optically thick to resonant Compton scattering. Photons propagate freely through the column until the photon frequency equals the local cyclotron frequency, at which point the radiation is scattered, much of it back toward the star. The radiation pressure in this regime is insufficient to stop the infall. Some of the scattered radiation heats the stellar surface around the base of the column, which adds a softer component to the spectrum. The partial blocking by the accretion column of X-rays from the surface produces a fan beam emission pattern. X-rays above the surface cyclotron frequency freely escape and are characterized by a pencil beam. Gravitational light bending, which is one of several general relativistic effects present in this problem, produces a pencil beam pattern of column-scattered radiation in the antipodal direction. The interaction of radiation with the accretion column produces a cyclotron feature that is strongly angle-dependent.