ON THE LENSE-THIRRING EFFECT IN ACCRETING BLACK HOLE SYSTEMS

Black holes are a natural prediction of the theory of General Relativity. Their gravitational field is strong enough to prevent light from escaping and to distort the space and the flow of time around them. Since light cannot escape, black holes can be detected when they are accreting mass from the surrounding gas or through the dynamics of nearby stars. Accretion of gas onto black holes is the most efficient mechanism that converts matter into energy. The investigation of this complex physical mechanism, in terms of both radiative emission and mechanical outflow, is crucial in order to understand the observations of accreting sources. In this context the Lense-Thirring or `frame dragging' effect becomes relevant since it warps the accretion disc around the black hole. The presence of warps within the disc can lead to its rigid precession around the black hole that results in a modulation of the emitted flux. Measuring the period and the duration of the quasi-periodic behaviour in the light curve, the black hole fundamental parameters (mass and spin) can be inferred. This thesis is devoted to the study of warps and rigid precession in accretion discs around black holes. The first part aims to give an introduction to the fundamental topics of the thesis: black holes, accretion discs and Lense-Thirring precession. The second is dedicated to tidal disruption event accretion discs formed around supermassive black holes. This contains an introduction to Tidal Disruption Events (TDEs) and a Chapter based on my first paper about LT precession of TDE accretion discs. The third part contains an introduction on the modelling of accretion discs in low mass X-ray binaries (LMXBs) around stellar mass black holes and two Chapters based on two papers I have co-authored that aim to constrain the black hole spin in LMXBs through the detection of type-C Quasi-Periodic Oscillations (QPOs) in the power density spectra of these sources. In the last Chapter I draw the conclusion and future perspectives of the work done in the thesis. The Appendix contains a detailed calculation of the spherical innermost stable orbit around a black hole and a general introduction to Smoothed Particle Hydrodynamic (SPH) simulations.