ON SPIN-UP SPIN-DOWN TORQUE REVERSALS IN DISC ACCRETING PULSARS/

The recent BATSE observations of the spin-up and spin-down of accretin g pulsars have shown that the standard formulation for the accretion t orque as proposed by Ghosh & Lamb may need to be revised. The observat ions indicate alternate spin-up and spin-down phases driven by torques of similar magnitude and typically larger than the mean torque. The v ariations of the torque in systems such as Cen X-3 are difficult to ex plain in terms of changes of the mass accretion rate. The implication is that the torque does not depend on the accretion rate as in the GL model. In this paper we argue that the observed changes in the spin ra te can result from stochastic transitions between two magnetospheric s tates. In particular, we show that intermediate magnetospheric systems are not admissible, because of a disc-induced magnetospheric instabil ity which exists in a star-disc magnetic interaction system. This expl ains why torque reversal occurs in disc accreting pulsars with similar magnitudes.