Hydromagnetics of advective accretion flows around black holes: Removal of angular momentum by large scale magnetic stresses

TL;DR

This paper demonstrates that large scale magnetic stresses can effectively remove angular momentum from advective accretion flows around black holes, providing an alternative to viscosity and explaining certain spectral states.

Contribution

It introduces a model showing magnetic stresses alone can transfer angular momentum efficiently in black hole accretion disks without alpha-viscosity, with implications for disk stability and outflows.

Findings

Magnetic stresses can replace viscosity in angular momentum transfer.

Field strength required is below equipartition, ensuring disk stability.

Faster outflows are associated with higher magnetic field gradients.

Abstract

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses 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 an angular momentum transfer could be equivalent to that of alpha-viscosity with alpha=0.01-0.08. Nevertheless, required field is well below its equipartition value, leading to a magnetically stable disk flow. This is essentially important in order to describe the hard spectral state of the sources, when the flow is non/sub-Keplerian. We show in our simpler 1.5-dimensional, vertically averaged disk model that 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. Finding efficient angular momentum transfer, in black hole disks, via magnetic stresses alone is very interesting, when the generic origin of alpha-viscosity is still being explored.