Spectral and temporal studies of accretion and ejection processes around supermassive black holes

In the centre of every major galaxy is a supermassive black hole. Some of these power an active galactic nucleus (AGN) in which the black hole is growing by accreting the luminous disc of material around it. As well as consuming matter, AGN can eject it in the form of powerful jets travelling at relativistic velocities. I present detailed studies of two narrow-line Seyfert 1 galaxies which exhibit powerful jets and high-energy gamma-ray emission. I explore the relationship between the disc and jet in these sources and suggest that their jets are relatively underpowered. Although AGN are too small and distant to be spatially resolved, temporal studies can reveal information about the processes occurring within the central engine. I conduct a temporal spectroscopic study of a hypervariable AGN which dimmed and rebrightened by a factor of three over four years. I demonstrate that the event is due to an intrinsic change within the accretion disc, and is not due to obscuration by an external body. Such dramatic variability poses a challenge to our current models of accretion discs. I draw attention to some new models which confront this problem. Finally, I place my findings in the context of the current literature and discuss some of its limitations and open questions. I highlight how future, planned observatories will help us to address these issues and deepen our understanding of AGN.