Dynamic accretion discs around supermassive black holes
thesisposted on 30.04.2021, 12:06 by Anagha Raj
Accretion discs around supermassive black holes act as the power houses of radiation to the most luminous, continuously emitting objects in the Universe; Active Galactic Nuclei (AGN). Accretion discs produce light, which we can observe by turning gravitational potential energy into heat via viscous torques. For discs around black holes, the available gravitational energy is the significant fraction of the rest mass energy of the orbiting matter. Simple, analytical models are capable of explaining the broad features of observed accreting black holes. However, recent observations have challenged existing theories by finding that AGN luminosities vary rapidly and with large amplitudes. In this thesis, we investigate the dynamics of warped discs with numerical simulations to see if such variability can be produced by these discs. Previous works have provided a criterion for warped discs to tear into discrete rings. We examine this possibility and explore how this may connect to observed disc behaviour. We demonstrate an agreement between our numerical results and the predicted criterion for disc tearing in warped discs around supermassive black holes. We also explore how our numerical analysis prove useful to illustrate the observational variability exhibited in AGN discs. In the later chapter, we show that often used initial conditions for accretion discs in simulations that employ Smoothed Particle Hydrodynamics (SPH) are not in dynamical equilibrium, and this leads to the formation of unwanted pressure waves that cannot be effectively damped at higher resolution. We propose a damping scheme, which uses an initial relaxing phase to remove these waves over a timescale comparable with the disc’s dynamical timescale.