Variability of black-hole accretion discs: a theoretical study

TL;DR

This thesis explores the variability in black-hole accretion discs through two theoretical models: disc instabilities from transonic flows and trapped oscillations due to relativistic effects, shedding light on high-frequency quasi-periodic phenomena.

Contribution

It introduces two novel classes of pseudo-relativistic models explaining high-frequency variability in black-hole accretion discs, linking instabilities and oscillations to relativistic flow properties.

Findings

Identification of disc instabilities linked to viscous overstability.

Analysis of inertial waves trapped by epicyclic frequency variations.

Impact of transonic flows on inertial mode propagation.

Abstract

This thesis investigates phenomena occurring in black-hole accretion discs which are likely to induce high-frequency quasi-periodic variability. Two classes of pseudo-relativistic theoretical models are studied. The first is based on the stability of transonic accretion flows and its connection to a disc instability that takes the form of propagating waves (viscous overstability). The second class of models looks at accretion-disc oscillations which are trapped due to the non-monotonic variation of the epicyclic frequency in relativistic flows. In particular, it focuses on inertial waves trapped below the maximum of the epicyclic frequency which are excited in deformed, warped or eccentric, discs. The influence of a transonic background on the propagation of these inertial modes is also investigated.