Accretion Disk Reversal and the Spin‐up/Spin‐down of Accreting Pulsars

We numerically investigate the hydrodynamics of accretion disk reversal andrelate our findings to the observed spin-rate changes in the accreting X-raypulsar GX~1+4. In this system, which accretes from a slow wind, the accretiondisk contains two dynamically distinct regions. In the inner part viscousforces are dominant and disk evolution occurs on a viscous timescale. In theouter part dynamical mixing of material with opposite angular momentum is moreimportant, and the externally imposed angular momentum reversal timescalegoverns the flow. In this outer region the disk is split into concentric ringsof material with opposite senses of rotation that do not mix completely butinstead remain distinct, with a clear gap between them. We thus predict thattorque reversals resulting from accretion disk reversals will be accompanied byminima in accretion luminosity.Comment: 13 pages, 7 figures, accepted for publication in Ap