The thickness of a weakly-magnetized accretion flow inside the last stable orbit of a Kerr black hole

TL;DR

This paper investigates the vertical structure and stability of magnetized accretion flows inside the last stable orbit of a Kerr black hole, revealing oscillations, increased thickness, and potential instabilities into thinner streams.

Contribution

It provides a theoretical model for the flow thickness influenced by magnetic fields and confirms complex vertical structures through numerical simulations.

Findings

Flow thickness oscillates around equilibrium due to magnetic forces.

Magnetically supported flows are unstable and may segregate into thinner streams.

Vertical structure is more complex than simple theoretical predictions.

Abstract

If accretion disc contains weak frozen-in entangled magnetic fields, their dynamical effect may be important inside the last stable orbit because of the decompression near the sonic point. Here, I consider the radial and vertical structure of a nearly free-falling flow inside the last stable orbit of a thin disc around a Kerr black hole. The thickness of such a flow is determined primarily by the vertical stress created by radial and azimuthal magnetic fields. The thickness is predicted to oscillate vertically around its equilibrium value determined by the magnetic field balance with gravity. For thin discs, this thickness is much larger than that of the accretion disc itself. Numerical simulations with HARM2d show the vertical structure is more complicated. In particular, magnetically supported disc seems to be unstable to segregation of matter into thinner streams with the vertical scale determined by thermal pressure or other processes.