Accretion disk reversal and the spin-up/spin-down of accreting pulsars

We numerically investigate the hydrodynamics of accretion disk reversal and relate our findings to the observed spin-rate changes in the accreting X-r ay pulsar GX 1 + 4. In this system, which accretes from a slow wind, the ac cretion disk contains two dynamically distinct regions. In the inner part v iscous forces are dominant, and disk evolution occurs on a viscous timescal e. In the outer part dynamical mixing of material with opposite angular mom entum is more important, and the externally imposed angular momentum revers al timescale governs the flow. In this outer region the disk is split into concentric rings of material with opposite senses of rotation that do not m ix completely but instead remain distinct, with a clear gap between them. W e thus predict that torque reversals resulting from accretion disk reversal s will be accompanied by minima in accretion luminosity.