Excitation of Trapped g-Mode Oscillations in Warped Disks around Black Holes

TL;DR

This study numerically investigates the resonant excitation of trapped g-mode oscillations in warped accretion disks around black holes, revealing conditions that enhance their growth and potential link to observed X-ray phenomena.

Contribution

It demonstrates that warped disk oscillations can be resonantly excited, with growth rates influenced by warp amplitude, black hole spin, and sound speed, advancing understanding of high-frequency QPOs.

Findings

Fundamental g-mode oscillations are excited near the resonance radius.

Growth rate increases with warp amplitude and black hole spin.

Growth rate decreases as sound speed increases.

Abstract

In order to study the origin of high-frequency quasi-periodic oscillations observed in X-ray binaries, Kato (2004) suggested a resonant excitation mechanism of disk oscillations in deformed disks. In this paper, we study numerically, following his formulation, whether trapped g-mode oscillations in a warped disk, where the warp amplitude varies with radius, can be excited by this mechanism. For simplicity, we adopt Newtonian hydrodynamic equations with relativistic expressions for the characteristic frequencies of disks. We also assume that the accretion disk is isothermal. We find that the fundamental modes of trapped g-mode oscillations with eigenfrequencies close to the maximum of epycyclic frequency are excited. The intermediate oscillations found are isolated in a narrow region around the resonance radius. After varying some parameters, we find that the growth rate increases as the warp amplitude or the black hole spin parameter increases, while it decreases as the sound speed increases.