Spin-reversed accretion as the cause of intermittent spindown in slow X-ray pulsars

It is shown that the magnitude and sign of the angular momentum captured by an X-ray pulsar from the its mass-losing binary companion are very sensitive to the distribution of the relative flow velocity (as well as the matter density) across the capture cross-section. Small fluctuations in the mass transfer progress can thus lead to a reversal in the direction of the accretion torque, providing an explanation for the spinup-spindown behaviour of some of the slow X-ray pulsars. The simultaneous measurement of pulse periods and X-ray fluxes during spindow episodes would help discriminate between spin-reversal and 'propeller-type' torques. (orig.)