X-ray and optical studies of active galactic nuclei

Active Galactic Nuclei are among the most highly energetic objects in the universe, but they are still poorly understood. Of particular interest at the moment is the existence and effects of massive accretion disks near the centre of AGN. Accretion disks have been invoked to explain both the UV and soft X-ray continuum in AGN and have also been used to try and explain the characteristics of the observed X-ray variability in AGN. This thesis describes both optical and X-ray observations of Active Galactic Nuclei. The optical data consists of a soft X-ray selected sample taken from the EXOSAT High Galactic Latitude Survey. In chapter 1, an extensive introduction details current theories about the continuum and line emission in AGN. Chapters 2-5 then go on to detail the results obtained from AGN identified from the EXOSAT HGLS, with detailed studies of both the continuum and line emission properties. The conclusion from the analysis is that the EXOSAT EGLS contain objects with a strong soft X-ray excess, and that the origin of the soft excess is from the high energy tail of an accretion disk. The edge-on disk model of Laor and Netzer (1990) is favoured. The emission lines indicate that these objects are in no way different from other AGN, and imply that the soft X-ray excess must be emitted anisotropically. There is also a slight suggestion of a change in the spectral characteristics of the AGN as a function of luminosity. The last chapter (chapter 6) concentrates on the X-ray variability of a nearby Seyfert galaxy NGC6814. The EXOSAT observations reported in this thesis are unique, since at least one of the observations shows evidence for periodic behaviour, particularly in the form of recurrent flares. Different models for the production of these flares are discussed.