Accretion disks: The magnetohydrodynamic powerhouse

Abstract

Accretion disks orbiting around compact objects such as white dwarfs, neutron stars and black holes are a central paradigm of high energy astrophysics. Accretion disks are believed to power systems such as x-ray binaries, active galactic nuclei, and quasars through the release of gravitational potential energy as gas spirals in. This infall requires the transfer of angular momentum outward from one fluid element to another. Thus the dynamics and evolution of accretion disks depend fundamentally upon this angular momentum transport process. Because accretion disks are composed of highly ionized conducting plasma with very high Reynolds number, the equations governing their evolution are those of (inviscid) magnetohydrodynamics (MHD). In disks the combination of a subthermal magnetic field and outwardly decreasing differential rotation leads to a powerful linear instability (the magnetorotational instability) which, in turn, generates MHD turbulence that produces a net Maxwell stress and the required angular momentum transport. The turbulence generates heat through dissipation at the viscous and resistive length-scales. Thus, accretion disks are systems where the power generation mechanism derives directly from a magnetohydrodynamic process. Fully global three-dimensional MHD simulations are now beginning to probe the properties of accretion disks from first principles.