LOCATION OF THE INNER RADIUS OF A MAGNETICALLY THREADED ACCRETION DISK

摘要

In models for disk accretion onto magnetized objects, the inner radius R_0 of the Keplerian disk is conventionally expressed in the form R_0 = ξr_A, where r_A is the Alfven radius for spherical accretion and the parameter ξ is usually taken to be 0.5. We point out that the value of ξ in fact depends on the fraction of the star's magnetic flux threading the disk, which is poorly known from theory: in general, ξ approx = 1.35η~(4/7), where η ≤ 1 denotes the threading coefficient. Application of the beat frequency model to binary X-ray pulsars showing quasi-periodic oscillations suggests strongly that η ～ 1 for these objects: the stellar dipole field essentially fully threads the disk and ξ approx = 1. When combined with improved accretion torque models characterized by critical fastness parameters near unity, the corrected values of ξ ≡ R_0/r_A should allow more reliable determinations of the accreting star's dipole moment and mass-radius relationship.