Mh. Vankerkwijk et al., WARPED DISKS AS A POSSIBLE ORIGIN OF TORQUE REVERSALS IN ACCRETION-POWERED PULSARS, The Astrophysical journal, 499(1), 1998, pp. 27-30
Enigmatic transitions between spin-up and spin-down have been observed
in several X-ray pulsars accreting matter via an accretion disk. In t
hese transitions, the torque changes sign but remains at nearly the sa
me magnitude. It has been noted previously that alternating prograde a
nd retrograde disk flows would explain many features of the torque rev
ersals, although it has been unclear how a stable retrograde disk coul
d 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 degrees. This region would thus become retrogra
de, leading to a negative torque. Accretion disk models can show such
behavior, if account is taken of a warping instability due to irradiat
ion. The resulting ''flip-overs'' of the inner parts of the disk can r
eproduce most characteristics of the observations, although it remains
unclear what sets the timescale on which the phenomenon occurs. If th
is model were correct, it would have a number of ramifications, for in
stance, that in the spin-down state, the X-ray source would mostly be
observed through the accretion disk.