Magnetohydrodynamics of accretion disks

Abstract

Accretion disks in close binary systems originate when mass overflow occurs from the primary star onto the compact star. When the compact star is a neutron star or a black hole the inner parts of the thin disk extend to the Alfvén radius respectively a few times the Schwarzchild radius. In the Keplerian rotating highly turbulent inner parts of the accretion disk magnetic fields are strongly amplified and expelled from the disk thus leading to the formation of a magnetically structured accretion disk corona, sandwiching the disk to which it is electrodynamically coupled. The magnetic energy supplied to the corona is radiated by inverse Compton scattering of soft X-ray photons produced in the disk by the viscous heating of the accreting matter. This may explain why certain X-ray sources show a very large fluctuating hard X-ray component. The interaction of the inner parts of an accretion disk with the magnetosphere around a neutron star leads to channeled accretion onto the magnetic poles, resulting in the phenomenon of X-ray pulsars with the associated spin variations due to angular momentum transfer. The interaction of disk coronal structures with the relatively weak magnetic fields of old fast spinning neutron stars lead to a new form of interaction around the so called beat frequency that can be used as a model for quasiperiodic oscillations in low-mass X-ray binaries.