Magnetically Arrested Disks and Origin of Poynting Jets: Numerical Study

TL;DR

This study uses numerical simulations to explore magnetically arrested disks (MADs) around black holes, revealing how magnetic fields influence accretion flows and lead to the formation of Poynting jets, with implications for observed spectral state changes.

Contribution

It provides the first detailed numerical analysis of MAD formation, magnetic field dynamics, and jet launching mechanisms in black hole accretion disks.

Findings

MAD forms when magnetic fields reach equipartition, disrupting the outer disk.

Poynting jets are generated with up to 1.5% efficiency of accretion energy.

Spiral flow modes may cause quasi-periodic oscillations in emitted radiation.

Abstract

The dynamics and structure of accretion disks, which accumulate the vertical magnetic field in the centers, are investigated using two- and three-dimensional MHD simulations. The central field can be built up to the equipartition level and disrupts a nearly axisymmetric outer accretion disk inside a magnetospheric radius, forming a magnetically arrested disk (MAD). In the MAD, the mass accretes in a form of irregular dense spiral streams and the vertical field, split into separate bundles, penetrates through the disk plane in low-density magnetic islands. The accreting mass, when spiraling inward, drags the field and twists it around the axis of rotation, resulting in collimated Poynting jets in the polar directions. These jets are powered by the accretion flow with the efficiency up to ~1.5% (in units \dot{M}c^2). The spiral flow pattern in the MAD is dominated by modes with low azimuthal wavenumbers m~1-5 and can be a source of quasi-periodic oscillations in the outgoing radiation. The formation of MAD and Poynting jets can naturally explain the observed changes of spectral states in Galactic black hole binaries. Our study is focused on black hole accretion flows; however, the results can also be applicable to accretion disks around nonrelativistic objects, such as young stellar objects and stars in binary systems.