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Abstract
Binary X-ray pulsars are strongly magnetic neutron stars accreting matter from a companion star. The strong magnetic fields of the neutron stars guide the accreting plasma to the magnetic poles, and X-ray pulsations result from the rotation of the beamed emission at the magnetic poles through the line of sight. In many systems, accretion proceeds via a Keplerian accretion disk. The differential rotation of the disk and star creates magnetic stresses that disrupt the Keplerian flow, and bring the plasma into corotation with the neutron star well above the stellar surface.
We report the results of a detailed numerical study of the interaction of a slim Keplerian accretion disk with the magnetic field of an aligned rotator. We show that the accretion flow generally exhibits a sonic point because of the large radial velocity the disk plasma acquires following the loss of centrifugal support caused by magnetic braking of the plasma in the accretion disk. Our model includes, for the first time, the effect of the rotation of the neutron star on the disk flow, and allows us to compute the location of the inner edge of the Keplerian flow and the torque on the neutron star as functions of the stellar spin frequency.
In a preliminary calculation, we assume that the poloidal component of the magnetic field interacting with the disk is dipolar; in an improved treatment, we include the screening effect of the electrical currents induced in the disk.
We model the field-aligned flow of plasma from the inner disk to the magnetic poles of the neutron star. We calculate the electrical current density in this region, and we estimate the screening effect of these currents.
We discuss the implications of our results for the spin frequency behavior of X-ray pulsars accreting from disks.
