TORQUE REVERSAL IN ACCRETION-POWERED X-RAY PULSARS

Accretion-powered X-ray pulsars 4U 1626-67, GX 1+4, and OAO 1657-415 h ave recently shown puzzling torque reversals. These reversals are char acterized by short timescales, on the order of days, nearly identical spin-up and spin-down rates, and very small changes in X-ray luminosit y. We propose that this phenomenon is the result of sudden dynamical c hanges in the accretion disks triggered by a gradual variation of mass accretion rates. These sudden torque reversals may occur at a critica l accretion rate similar to 10(15)-10(16)g s(-1) when the system makes a transition from (to) a primarily Keplerian flow to (from) a substan tially sub-Keplerian, radial advective flow in the inner disk. For sys tems near spin equilibrium, the spin-up torques in the Keplerian state are slightly larger than the spin-down torques in the advective state , in agreement with observation. The abrupt reversals could be a signa ture of pulsar systems near spin equilibrium with the mass accretion r ates modulated on a timescale of a year or longer near the critical ac cretion rate. It is interesting that cataclysmic variables and black h ole soft X-ray transients change their X-ray emission properties at ac cretion rates similar to the pulsars' critical rate. We speculate that the dynamical change in pulsar systems shares a common physical origi n with white dwarf and black hole accretion disk systems.