ON THE DRAMATIC SPIN-UP SPIN-DOWN TORQUE REVERSALS IN ACCRETING PULSARS/

Dramatic torque reversals between spin-up and: spin-down have been obs erved in half of the persistent X-ray pulsars monitored by the BATSE a ll-sky monitor on the Compton Gamma Ray Observatory. Theoretical model s developed to explain early pulsar timing data can explain spin-down torques via a disk-magnetosphere interaction if the star nearly corota tes with the inner accretion disk. To produce the observed BATSE torqu e reversals, however, these equilibrium models require the disk to alt ernate between two mass accretion rates, with (M) over dot(+/-) produc ing accretion torques of similar magnitude but always of opposite sign . Moreover, in at least one pulsar (GX 1+4) undergoing secular spin-do wn, the neutron star spins down faster during brief (similar to 20 day ) hard X-ray flares-this is opposite the correlation expected from sta ndard theory, assuming that BATSE pulsed flux increases with mass accr etion rate. The 10 day to 10 yr intervals between torque reversals in these systems are much longer than any characteristic magnetic or visc ous timescale near the inner disk boundary and are more suggestive of a global disk phenomenon. We discuss possible explanations of the obse rved torque behavior. Despite the preferred sense of rotation defined by the binary orbit, the BATSE observations are surprisingly consisten t with an earlier suggestion for GX 1+4: the disks in these systems so mehow alternate between episodes of prograde and retrograde rotation. We are unaware of any mechanism that could produce a stable retrograde disk in a binary undergoing Roche lobe overflow, but such flip-flop b ehavior does occur in numerical simulations of wind-fed systems, One p ossibility is that the disks in some of these binaries are fed by an X -ray-excited wind.