The azimuthal structure of magnetically arrested disks during flux eruption events

¹ Section of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens, University Campus, Zografos GR-15784, Athens, Greece
² Research Center for Astronomy and Applied Mathematics, Academy of Athens, Soranou Efessiou 4, GR-11527 Athens, Greece

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Received: 23 May 2025
Accepted: 15 April 2026

Abstract

Context. Magnetically arrested disks (MADs) are highly dynamic astrophysical systems characterized by strong variability and transient phenomena such as magnetic flux eruption events.

Aims.We investigated the azimuthal structure of the equatorial inner accretion flow during flux eruption events and propose a physical mechanism for the formation and outward transport of vertical magnetic flux tubes.

Methods. We analyzed data from a standard 3D general-relativistic magnetohydrodynamic simulation, focusing on equatorial slices in order to examine the details and evolution of the azimuthal structure of the accreting matter.

Results. 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 found 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 because features with a large angular size emerge 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 outward due to magnetic buoyancy.

Conclusions. 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.