Torque Reversal in Accretion-powered X-Ray Pulsars

Accretion-powered X-ray pulsars 4U 1626-67, GX 1+4, and OAO 1657-415 haverecently shown puzzling torque reversals. These reversals are characterized byshort time scales, on the order of days, nearly identical spin-up and spin-downrates, and very small changes in X-ray luminosity. We propose that thisphenomenon is the result of sudden dynamical changes in the accretion diskstriggered by a gradual variation of mass accretion rates. These sudden torquereversals may occur at a critical accretion rate $\sim$$10^{15}-10^{16}g~s^{-1}$ when the system makes a transition from (to) aprimarily Keplerian flow to (from) a substantially sub-Keplerian, radialadvective flow in the inner disk. For systems near spin equilibrium, thespin-up torques in the Keplerian state are slightly larger than the spin-downtorques in the advective state, in agreement with observation. The abruptreversals could be a signature of pulsar systems near spin equilibrium with themass accretion rates modulated on a time scale of a year or longer near thecritical accretion rate. It is interesting that cataclysmic variables and blackhole soft X-ray transients change their X-ray emission properties at accretionrates similar to the pulsars' critical rate. We speculate that the dynamicalchange in pulsar systems shares a common physical origin with white dwarf andblack hole accretion disk systems.Comment: 11 pages, parameter corrections, ApJ