The azimuthal structure of magnetically arrested disks during flux eruption events

TL;DR

This study uses 3D GRMHD simulations to analyze how magnetic flux eruption events influence the azimuthal structure of accretion disks near black holes, highlighting the role of magnetic flux reconfiguration and non-axisymmetric features.

Contribution

It provides a detailed quantitative analysis of the azimuthal structure and evolution of magnetically arrested disks during flux eruptions, emphasizing magnetic flux reconfiguration processes.

Findings

Non-axisymmetric features are enhanced during flux eruptions.

Low azimuthal modes (m=1,2) dominate near the black hole.

Vertical magnetic flux bundles form and are expelled via magnetic buoyancy.

Abstract

We analyze data from a standard 3D general-relativistic magnetohydrodynamics (GRMHD) simulation, focusing on equatorial slices in order to examine the details and the evolution of the azimuthal structure of the accreting matter. During flux eruption events, the non-axisymmetric features of the equatorial inner accretion disk are considerably enhanced, with this enhancement being more prominent close to the black hole. Our analysis of the azimuthal structure of the equatorial accretion disk finds that the matter distribution in the vicinity of the horizon is dominated by low azimuthal mode numbers, specifically by the $m = 2$, and $m = 1$ modes, indicating that the non-axisymmetry of the disk during flux eruption events is enhanced due to the emergence of features with a large angular size on the equatorial plane. Our results suggest that the morphology of the equatorial accretion flow close to the black hole is mainly determined by the formation and motion of vertical magnetic flux bundles. These bundles are formed when the initially horizontal magnetic field reconnects into a vertical configuration, effectively detaching from the black hole horizon. This reconnection occurs in a low-density, highly magnetized region on the equatorial plane that expands over time as more field lines undergo vertical reconfiguration. The resulting vertical flux tubes, filled with low-density plasma, are then transported outwards due to magnetic buoyancy. Our results present a detailed quantitative description of the morphology of MADs and of its evolution during flux eruptions, complemented by a description of the physical process by which excess magnetic flux is detached from the black hole, vertically reconfigured, and expelled.