WARPED DISKS AS A POSSIBLE ORIGIN OF TORQUE REVERSALS IN ACCRETION-POWERED PULSARS

Citation
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
Citations number
23
Categorie Soggetti
Astronomy & Astrophysics
Journal title
ISSN journal
0004637X
Volume
499
Issue
1
Year of publication
1998
Part
2
Pages
27 - 30
Database
ISI
SICI code
0004-637X(1998)499:1<27:WDAAPO>2.0.ZU;2-B
Abstract
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.