X-ray spectral variability as a probe of active galactic nuclei.

This thesis is concerned with how studies of variability in the X-ray spectrum of Active Galactic Nuclei (AGN) can place constraints on physical models of the X- ray emission and absorption mechanisms in such objects. An underlying theme of the thesis is to place emphasis on modelling individual objects as well as a class of objects and to this end the Seyfert I galaxy, NGC 4151, has been targeted for an individual case study. NGC 4151 is ideally suited for the present purpose as it has been extensively studied in all wavebands, from radio to X-ray energies and has long been known to exhibit X-ray spectral variability. We first review the diagnostics of AGN which are of contemporary importance, mainly due to studies of the optical/ UV emission-line spectrum, and outline how the study of X-ray spectral variability may be used as an additional diagnostic tool. We then present the results of an extensive (~ 5.5 year) monitoring campaign of NGC 4151 with EXOSAT and Ginga. The medium energy X-ray flux exhibited large amplitude variability accompanied by significant spectral variations. We show that these spectral changes can be attributed to both changes in the configuration of the X-ray absorbing material in the nucleus (independent of the continuum level) and in the spectral index of X-ray power law which is correlated with the 2-10 keV flux. We then investigate some theoretical models of spectral index variability and of complex and variable absorption. The implications of these models for AGN in general and NGC 4151 in particular are discussed. We also present X-ray observations of two other AGN, namely ESO 103-G35 and QSO 1821+643, both of which exhibit significant X-ray spectral variability. The results are discussed in terms of contemporary models.