Probing the central engine of active galactic nuclei using time delay studies

Understanding the physics of accretion onto black holes, and the role these cosmic engines play in shaping the universe, is one of the major challenges of contemporary astrophysics research. In this thesis I present several studies, aiming to better understand the variable emission we observe in accreting black holes. The focus of these studies are highly variable Narrow Line Seyfert 1 galaxies in the nearby universe. I start by summarising the current field of research in an introductory chapter. This is followed by a more technical chapter, outlining the analysis methods used in any subsequent chapters. This thesis contains five science chapters which are summarised below. I end with some concluding remarks and a discussion about the future prospects of the field. In chapter 3 we present a study of correlated UV and X-ray variability in the NGC 4051, where we found evidence for the reprocessing of X-ray photons in the disc. In chapter 4 we present a study of the time delays between a soft and hard X-ray band as a function of source flux in NGC 4051. We found a strong flux dependence on the time delays, and through analytical modelling we found reprocessing of the primary power law emission close to the central source. In chapter 5 we study the X-ray time delays in a sample of objects, and found a significant flux dependence in only one other source, Ark 564. In chapter 6 we study the energy and flux dependence of the X-ray timing properties in NGC 4051. A strong flux dependence on the lag energy is found and simple modelling performed. In chapter 7 we study the frequency and energy dependent lags in PG 1244+026, and found high frequency lags from both reflection and thermal reprocessing.