Removal of angular momentum by strong magnetic field stresses in advective accretion flows around black holes

TL;DR

This paper demonstrates that strong magnetic stresses can effectively remove angular momentum in advective accretion flows around black holes, potentially replacing traditional viscosity mechanisms and influencing outflow rates and spectral states.

Contribution

It introduces a model where magnetic stresses, much stronger than those needed for magnetorotational instability, can transfer angular momentum without alpha-viscosity in black hole accretion flows.

Findings

Magnetic stresses can match alpha-viscosity in angular momentum transfer.

Stronger magnetic fields increase angular momentum transfer and outflow rates.

Magnetic stress-driven transfer impacts spectral states of black hole sources.

Abstract

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses, arisen by fields much stronger than that required for magnetorotational instability, in geometrically thick, advective, sub-Keplerian accretion flows around black holes in steady-state, in the complete absence of alpha-viscosity. The efficiency of such angular momentum transfer via Maxwell stress, with the field well below its equipartition value, could be equivalent to that of alpha-viscosity, arisen via Reynolds stress, with $\alpha=0.01-0.08$. We find in our simpler vertically averaged advective disk model that stronger the magnetic field and/or larger the vertical-gradient of azimuthal component of magnetic field, stronger the rate of angular momentum transfer is, which in turn may lead to a faster rate of outflowing matter, which has important implications to describe the hard spectral states of black hole sources. When the generic origin of alpha-viscosity is still being explored, mechanism of efficient angular momentum transfer via magnetic stresses alone is very interesting.