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

Enigmatic transitions between spin-up and spin-down have been observed inseveral X-ray pulsars accreting matter via an accretion disk. In thesetransitions, the torque changes sign but remains at nearly the same magnitude.It has been noted previously that alternating prograde and retrograde diskflows would explain many features of the torque reversals, although it has beenunclear how a stable retrograde disk could be formed. We suggest that thereversals may be related to the disk at times being warped to such an extentthat the inner region becomes tilted by more than 90 degrees. This region wouldthus become retrograde, leading to a negative torque. Accretion disk models canshow such behavior, if account is taken of a warping instability due toirradiation. The resulting `flip-overs' of the inner parts of the disk canreproduce most characteristics of the observations, although it remains unclearwhat sets the timescale on which the phenomenon occurs. If this model werecorrect, it would have a number of ramifications, for instance that in thespin-down state the X-ray source would mostly be observed through the accretiondisk.Comment: 6 pages, 1 figure, aas2pp4.sty, accepted by ApJ Letters. Differs by small clarifications only from previous (submitted) versio