On an excitation mechanism for trapped inertial waves in discs around black holes

TL;DR

This paper investigates a non-linear coupling mechanism involving disc warping or eccentricity that can efficiently excite trapped inertial waves in accretion discs around black holes, potentially explaining high-frequency QPOs.

Contribution

It details a non-linear excitation mechanism for inertial waves in black hole accretion discs, emphasizing the role of global deformations and calculating growth rates based on black hole spin and disc properties.

Findings

The coupling mechanism can efficiently excite inertial waves if disc deformations reach the inner disc.

Growth rates depend on black hole spin and disc characteristics.

Global warping or eccentricity is crucial for wave amplification.

Abstract

According to one model, high-frequency quasi-periodic oscillations (QPOs) can be identified with inertial waves, trapped in the inner regions of accretion discs around black holes due to relativistic effects. In order to be detected, their amplitudes need to reach large enough values via some excitation mechanism. We work out in detail a non-linear coupling mechanism suggested by Kato, in which a global warping or eccentricity of the disc has a fundamental role. These large-scale deformations combine with trapped modes to generate `intermediate' waves of negative energy that are damped as they approach either their corotation resonance or the inner edge of the disc, resulting in amplification of the trapped waves. We determine the growth rates of the inertial modes, as well as their dependence on the spin of the black hole and the properties of the disc. Our results indicate that this coupling mechanism can provide an efficient excitation of trapped inertial waves, provided the global deformations reach the inner part of the disc with non-negligible amplitude.