The Large-scale Magnetic Field of a Thin Accretion Disk with Outflows

Abstract

The large-scale magnetic field threading an accretion disk plays an important role in launching jets/outflows. The field may probably be advected inward by the plasma in the accretion disk from the ambient environment (interstellar medium or a companion star). It has been suggested that the external field can be efficiently dragged inward in a thin disk with magnetic outflows. We construct a self-consistent global disk-outflow model in which the large-scale field is formed by the advection of the external field in the disk. The outflows are accelerated by this field corotating with the disk, which carries away most of the angular momentum of the disk and causes its structure to become significantly different from the conventional viscous disk structure. We find that the magnetic field strength in the inner region of the disk can be several orders of magnitude higher than the external field strength for a geometrically thin disk with H / R  ∼0.1 if the ratio of the gas to magnetic pressure β out  ∼ 10 2 at the outer edge of the disk. The outflow velocity shows a layer-like structure, i.e., it decreases with radius where it is launched. The outflow can be accelerated up to ∼0.2–0.3 c from the inner region of the disk, and the mass-loss rate in the outflows is ∼10%–70% of the mass accretion rate at the outer radius of the disk, which may account for the fast outflows that are observed in some active galactic nuclei.