Simulating spherical and extreme accretion events around black holes

Abstract

This thesis investigates the physics of two extreme types of accretion flows around black holes: Bondi-like accretion flow and super-Eddington accretion flow. By performing 3D general relativistic magnetohydrodynamics (GRMHD) simulations, I examine the structure and the dynamical evolution of these accretion flows and their jets, winds and emissions.

For the first part of the study, I conduct GRMHD simulations of accretion flows with zero or very low specific angular momenta. The results demonstrate that the accretion flow needs to have an initial specific angular momentum above a certain threshold to robustly sustain the magnetically arrested disk state, which is crucial to launch very powerful jets at more than one hundred percent energy efficiency. Interestingly, an accretion flow with zero gas angular momentum can still launch intermittent jets with a smaller energy efficiency. The findings shed light on why jets can be produced from various types of black hole accretion systems in which the gas is expected to have low angular momentum.

For the second part of the study, I perform radiation GRMHD simulations to study super-Eddington accretion disks around massive black holes at different Eddington ratios ranging from a few to a few tens. The results demonstrate that the inflow--outflow structure remains robust as the accretion rate varies, while the inflow--outflow gas density generally increases with the accretion rates. The simulated disks are then post-processed, and their spectra are used to understand the emissions from tidal disruption events, in which stars are destroyed by the tidal force from massive black holes, and their debris is fed to the black hole at super-Eddington rates.