Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs

TL;DR

This paper investigates how corotational damping affects diskoseismic c-modes in black hole accretion discs, showing that damping is generally weak but significant enough to influence mode observability.

Contribution

It provides the first quantitative calculation of corotational damping rates for c-modes using the WKB approximation, highlighting their dependence on physical parameters.

Findings

Damping rates are typically less than 10% of mode frequency.

Wave absorption at corotation resonance causes the damping.

Strong excitation mechanisms are required for mode detection.

Abstract

Diskoseismic c-modes in accretion discs have been invoked to explain low-frequency variabilities observed in black-hole X-ray binaries. These modes are trapped in the inner-most region of the disc and have frequencies much lower than the rotation frequency at the disc inner radius. We show that because the trapped waves can tunnel through the evanescent barrier to the corotational wave zone, the c-modes are damped due to wave absorption at the corotation resonance. We calculate the corotational damping rates of various c-modes using the WKB approximation. The damping rate varies widely depending on the mode frequency, the black hole spin parameter and the disc sound speed, and is generally much less than 10% of the mode frequency. A sufficiently strong excitation mechanism is needed to overcome this corotational damping and make the mode observable.