Session 100 - Pulsars & X-ray Binaries.
Oral session, Friday, January 09
Monroe,

## [100.02] Explaining Torque-Reversing X-ray Pulsars

I. Yi (IAS/Princeton)

Accretion-powered X-ray pulsars such as 4U 1626-67, GX 1+4, and OAO 1657-415 have shown abrupt torque reversals which are difficult to explain within the conventional models. The reversals are characterized by (i) short reversal time scales, (ii) nearly identical spin-up and spin-down rates, and (iii) small X-ray luminosity changes. A dramatic spectral transition has been reported in 4U 1626-67 and a mysterious torque-flux correlation has been observed in GX 1+4. We show that the observed phenomenon is well accounted for by the accretion disk transition triggered by a gradual, small amplitude modulation of mass accretion rate. The sudden reversals occur at a rate \sim 10^16-17 g/s when the accretion flow makes a transition from (to) a primarily Keplerian flow to (from) a substantially sub-Keplerian, radial advective flow. The transition time scale is likely to be shorter than days, the required accretion rate change is at the level of a few \times 10 percent, and the spin-up rate is slightly larger than the spin-down rate. The abrupt reversal is a signature of a pulsar system near spin equilibrium with small mass accretion rate modulations near the critical accretion rate on the time scale of years. We suggest possible explanations for the spectral transition and the torque-flux correlation. The accretion disk transition is similar to those in black hole transients and cataclysmic variables, which strongly suggest a common physical origin.