On the nature and role of absorption and continuum reprocessing in the X-ray spectra of obscured Seyfert galaxies

This thesis presents an investigation of the X-ray properties of four nearby galaxies, NGC 4151, NGC 7582, Mrk 3 and NGC 4945, all of which harbour a bright, heavily obscured Seyfert nucleus. It explores the implications of the results from these sources for heavily obscured active galactic nuclei (AGN) in general. The work focuses largely on data from the XMM-Newton X-ray satellite, concentrating initially on hard X-ray spectra from the EPIC CCD cameras. The analysis emphasises the complex nature of the X- ray absorption in NGC 4151 and highlights the massive impact that absorption has on the observed X-ray spectrum and variability of all four sources. The range of column densities exhibited (1023 cm-2 to >1024 cm-2) encompasses both Compton-thin, continuum-dominated sources (such as NGC 4151) and Compton-thick, reflection-dominated sources (e.g. Mrk 3 and NGC 4945). The often-subtle role that Compton reflection plays in the X-ray spectrum and variability of these sources is discussed in de tail. The source spectra, in each case, contain an intrinsically narrow, neutral, iron Ka line emission and a sharp iron K edge, adding to the growing body of evidence which suggests that iron reprocessing features are ubiquitous constituents of the hard X-ray spectra of AGN. The analysis of NGC 4945 demonstrates the enormous impact that a nuclear starburst can have on the X-ray properties of AGN. A preliminary analysis of the soft X-ray spectrum of NGC 4151, revealed by the high spectral resolution XMM-Newton RGS instruments, shows the spectrum to be completely dominated by line emission and radiative recombination continua from hydrogen-like and helium-like ions. It is argued that the soft emission originates in photoionized and photoexcited gas located in X-ray/optical ionization cones. The observational findings presented in this thesis are all generally consistent with the unified AGN picture, in which the major differences between the sources are produced by different orientations with respect to our line-of-sight.