Abstract

Accretion disks are the preferred central engine for AGN, but theoretical progress has long been hampered by the unknown nature of the angular momentum transport mechanism. An obstacle preventing the general acceptance of turbulent disk theories has been the intransigent stability of thin Keplerian disks to hydrodynamic perturbations. This difficulty is overcome by the result discussed here, that a weak magnetic field renders a disk locally unstable whenever the angular velocity decreases outward. The salient and remarkable feature that distinguishes this instability is that the maximal growth rate (which is of order the angular velocity) is independent of the strength of the field, even as the latter vanishes. The instability can be derived straightforwardly from the equations of orbital mechanics and a spring-like magnetic interaction. We list several key questions regarding the instability's implications for realistic disks, and review the results from two-dimensional numerical simulations.

Introduction

Many of the scenarios created to explain the spectra from Active Galactic Nuclei (AGN) involve, either implicitly or explicitly, the release of gravitational energy through accretion. The classical paper of Lynden-Bell (1969) was the first to suggest that the heart of an active nucleus consisted of an accretion disk surrounding a supermassive black hole. The appeal of this model (and its direct descendents) has always been one of energetics: no other proposed mechanism is as efficient and as compact. Gravitational potential energy release is hard to beat as an energy production mechanism.