Warped Disks as a Possible Origin of Torque Reversals in Accretion-powered Pulsars

Enigmatic transitions between spin-up and spin-down have been observed in several X-ray pulsars accreting matter via an accretion disk. In these transitions, the torque changes sign but remains at nearly the same magnitude. It has been noted previously that alternating prograde and retrograde disk flows would explain many features of the torque reversals, although it has been unclear how a stable retrograde disk could be formed. We suggest that the reversals may be related to the disk at times being warped to such an extent that the inner region becomes tilted by more than 90°. This region would thus become retrograde, leading to a negative torque. Accretion disk models can show such behavior, if account is taken of a warping instability due to irradiation. The resulting "flip-overs" of the inner parts of the disk can reproduce most characteristics of the observations, although it remains unclear what sets the timescale on which the phenomenon occurs. If this model were correct, it would have a number of ramifications, for instance, that in the spin-down state, the X-ray source would mostly be observed through the accretion disk.