Towards a New Standard Theory for Astrophysical Disk Accretion

TL;DR

This paper reviews recent advances in black hole accretion disk theory, emphasizing the importance of magnetic fields in angular momentum transport, which impacts disk structure and emission spectra.

Contribution

It introduces a new perspective on the role of large-scale MHD torques in angular momentum transport, modifying traditional accretion disk models.

Findings

Vertical MHD stresses can dominate angular momentum transport.

Magnetically-driven outflows make disks dimmer and redder.

Modified disk spectra impact black hole accretion interpretations.

Abstract

We briefly review recent developments in black hole accretion disk theory, placing new emphasis on the vital role played by magnetohydrodynamic (MHD) stresses in transporting angular momentum. The apparent universality of accretion-related outflow phenomena is a strong indicator that vertical transport of angular momentum by large-scale MHD torques is important and may even dominate radial transport by small-scale MHD turbulence. This leads to an enhanced overall rate of angular momentum transport and allows accretion of matter to proceed at an interesting rate. Furthermore, we argue that when vertical transport is important, the radial structure of the accretion disk is modified and this affects the disk emission spectrum. We present a simple model demonstrating that energetic, magnetically-driven outflows give rise to a disk spectrum that is dimmer and redder than a standard accretion disk accreting at the same rate. We briefly discuss the implications of this key result for accreting black holes in different astrophysical systems.