Black hole accretion and jet phenomena explored via general relativistic magnetohydrodynamic simulations

In recent years, General Relativistic Magnetohydrodynamic (GRMHD) simulations, combined with multiwavelength observations have provided critical insights into the nature of radiation from accreting black holes. These simulations have revealed a particularly interesting magnetically arrested disk (MAD) regime whereby the accretion is choked by the strong horizon-penetrating magnetic field. The higher magnetic flux characteristic of the MAD regime leads to new dynamics, including interchange-type accretion modes and flux eruptions governed by plasmoid dominated magnetic reconnection. Polarisation measurements by the Event Horizon Telescope (EHT) from the supermassive black holes M87* and Sgr A* favour MAD states. However, nearly all MAD models exhibit greater 230 GHz flux variability than seen in historical observations of Sgr A*. In this thesis, I investigate the dynamics of MAD GRMHD simulations and study the (sub)millimetre variability of Sgr A* focusing on the poorly understood electron thermodynamics. In addition, I analyse the multiwavelength signatures of a black hole X-ray binary outburst simulation to understand how synchrotron emission from the disc, jet, and their interfaces contribute to the X-ray emission and potentially influence observables such as polarisation.