Author Description

The most extreme physical conditions of space-time in the Universe happen in the vicinity of accreting black holes, which make them the perfect laboratory for testing extreme physics theories. The present thesis investigates accretion processes using radiation as a tracer of the physics occurring very close to the accreting black holes as well as far into the jets. It provides a means to understand the mechanism of the most powerful accelerator engines known in the Universe. Specifically, I study the importance of radiative processes in general relativistic magneto-hydrodynamic (GRMHD) simulations on the dynamics of the accretion flow around Sgr A* and examine the effects a self-consistent treatment of radiative cooling in GRMHD simulations has on the simulated spectra. I also discuss the contribution of the protons to the high-energy emission from jets of accreting black holes and present a new semi-analytic spectral jet model. This spectral model is then applied to analyse quasi-simultaneous observations from radio to the soft gamma-rays of Cygnus X-1.