Evolution of the inner radius of the accretion disk in the X-ray pulsar A0535+26

In disk-fed X-ray pulsars, the inner radius R(0) of the accretion disk is conventionally expressed in terms of the Alfven radius R(A) for spherical accretion, R(0) = xi R(A). The value of the dimensionless parameter xi and its dependence on the accretion rate are poorly known. Applying the beat frequency model and accretion torque theory to the transient X-ray pulsar A0535+26, in which the spin variation and the quasi-periodic oscillations were measured simultaneously during a ''giant'' outburst, we calculate the value of xi throughout the outburst, and find it generally close to unity, independent of the accretion rate, suggesting that the stellar field nearly fully threads the disk. It is also shown that during the initial rise of the outburst, xi increased from similar to 0.6 to similar to 0.9. We believe that this phenomenon reflects the transition of the accretion process of the neutron star from spherical accretion before the outburst to disk accretion during the outburst.