Prof Steven Balbus FRS, FInstP

Abstract:

We analyze three-dimensional magnetohydrodynamic simulations of a nonradiative accretion flow around a black hole using a pseudo-Newtonian potential. The flow originates from a torus initially centered at 100 gravitational (Schwarzschild) radii. Accretion is driven by turbulent stresses generated self-consistently by the magnetorotational instability. The resulting flow has three well-defined dynamical components: a hot, thick, rotationally dominated Keplerian disk; a surrounding magnetized corona with vigorous circulation and outflow; and a magnetically confined jet along the centrifugal funnel wall. Inside 10 gravitational radii, the disk becomes very hot, more toroidal, and highly intermittent. These results contrast sharply with quasi-spherical, self-similar viscous models. There are no significant dynamical differences between simulations that include resistive heating and those that do not. We conclude by deducing some simple radiative properties of our solutions, and apply the results to the accretion-powered Galactic center source Sgr A*.

Abstract:

We present a global magnetohydrodynamic (MHD) three-dimensional simulation of a nonradiative accretion flow originating in a pressure-supported torus. The evolution is controlled by the magnetorotational instability, which produces turbulence. The flow forms a nearly Keplerian disk. The total pressure scale height in this disk is comparable to the vertical size of the initial torus. Gas pressure dominates near the equator; magnetic pressure is more important in the surrounding atmosphere. A magnetically dominated bound outflow is driven from the disk. The accretion rate through the disk exceeds the final rate into the hole, and a hot torus forms inside 10rg. Hot gas, pushed up against the centrifugal barrier and confined by magnetic pressure, is ejected in a narrow, unbound, conical outflow. The dynamics are controlled by magnetic turbulence, not thermal convection, and a hydrodynamic α-model is inadequate to describe the flow. The limitations of two-dimensional MHD simulations are also discussed.