Angular Momentum Transport in Thin Magnetically Arrested Disks

TL;DR

This paper investigates how magnetic Rayleigh-Taylor instability influences angular momentum transport in thin magnetically arrested disks, revealing turbulence-driven stress mechanisms despite suppressed MRI activity.

Contribution

It demonstrates that magnetic RT instability induces turbulence that dominates angular momentum transport in thin MADs, contrasting with traditional MRI-driven models.

Findings

Magnetic RT instability creates low-density bubbles affecting disk dynamics.

Turbulent magnetic fields primarily drive angular momentum transport.

MRI suppression does not prevent turbulence from facilitating accretion.

Abstract

In accretion disks with large-scale ordered magnetic fields, the magnetorotational instability (MRI) is marginally suppressed, so other processes may drive angular momentum transport leading to accretion. Accretion could then be driven by large-scale magnetic fields via magnetic braking, but large-scale magnetic flux can build-up onto the black hole and within the disk leading to a magnetically-arrested disk (MAD). Such a MAD state is unstable to the magnetic Rayleigh-Taylor (RT) instability, which itself leads to vigorous turbulence and the emergence of low-density highly-magnetized bubbles. This instability was studied in a thin (ratio of half-height H to radius R, $H/R \approx 0.1$) MAD simulation, where it has a more dramatic effect on the dynamics of the disk than for thicker disks. We find that the low-density bubbles created by the magnetic RT instability decrease the stress (leading to angular momentum transport) in the disk rather than increasing magnetic torques. Indeed, we find that the dominant component of the stress is due to turbulent magnetic fields, despite the suppression of the axisymmetric MRI and the dominant presence of large-scale magnetic fields. This suggests that the magnetic RT instability plays a significant role in driving angular momentum transport in MADs.