An Accretion-Ejection Instability in magnetized disks

TL;DR

This paper introduces an instability in magnetized accretion disks where spiral density waves interact with Rossby vortices, potentially explaining low-frequency QPOs and contributing to wind or jet formation.

Contribution

It identifies a new accretion-ejection instability driven by magnetic stresses, linking spiral density waves, Rossby vortices, and Alfven wave emission in magnetized disks.

Findings

Instability occurs in disks with moderate vertical magnetic fields.

Spiral density waves exchange angular momentum with Rossby vortices.

The instability may explain low-frequency QPOs in X-ray binaries.

Abstract

We present an instability occurring in the inner part of disks threaded by a moderately strong vertical (poloidal) magnetic field. Its mechanism is such that a spiral density wave in the disk, driven by magnetic stresses (rather than self-gravity as in galactic spirals), becomes unstable by exchanging angular momentum with a Rossby vortex it generates at its corotation radius. This angular momentum can then ``leak'' as Alfven waves emitted toward the corona of the disk thus providing, as an element of the accretion process, an energetic source for a wind or a jet. As galactic spirals, this instability forms low azimuthal wavenumber, standing spiral patterns which might provide an explanation for low-frequency QPOs in low-mass X-ray binaries.