Corotation Resonance and Diskoseismology Modes of Black Hole Accretion Disks

TL;DR

This paper investigates how corotation resonance influences non-axisymmetric g-mode oscillations in thin accretion disks around black holes, showing that most observed modes are unaffected by the resonance due to their trapping regions.

Contribution

It provides a detailed analysis of corotation resonance effects on diskoseismology modes using a generalized WKB approach, clarifying when and how these resonances impact mode stability and observability.

Findings

Corotation resonance affects only some non-axisymmetric g-modes within their capture zones.

Most observed g-modes are unaffected by corotation resonance because it lies outside their trapping regions.

Viscosity can induce mode growth through secular instability at the corotation resonance.

Abstract

We demonstrate that the corotation resonance affects only some non-axisymmetric g-mode oscillations of thin accretion disks, since it is located within their capture zones. Using a more general (weaker radial WKB approximation) formulation of the governing equations, such g-modes, treated as perfect fluid perturbations, are shown to formally diverge at the position of the corotation resonance. A small amount of viscosity adds a small imaginary part to the eigenfrequency which has been shown to induce a secular instability (mode growth) if it acts hydrodynamically. The g-mode corotation resonance divergence disappears, but the mode magnitude can remain largest at the place of the corotation resonance. For the known g-modes with moderate values of the radial mode number and axial mode number (and any vertical mode number), the corotation resonance lies well outside their trapping region (and inside the innermost stable circular orbit), so the observationally relevant modes are unaffected by the resonance. The axisymmetric g-mode has been seen by Reynolds & Miller in a recent inviscid hydrodynamic accretion disk global numerical simulation. We also point out that the g-mode eigenfrequencies are approximately proportional to m for axial mode numbers |m|>0.