Seyfert galaxies as x-ray sources.

Using data from the Leicester University Ariel V Sky Survey Instrument (SSI), the work reported in this thesis establishes that Seyfert galaxies form a class of extragalactic x-ray source. The properties of Seyfert galaxies are reviewed and it is shown how standard arguments lead to a model of the structure of Seyfert galaxy nuclei containing three regions characterised by three types of emission: forbidden line, permitted line and 'non-thermal' continuum. Radio, infrared and optical parameters believed to derive from these three regions are then compared with x-ray flux densities from the SSI in an attempt to locate the source of x-ray emission. It is found that x-ray flux density correlates only with parameters describing the permitted line and continuum regions. Standard arguments presented earlier show that these regions are very small (=0.01 - 1pc diameter). Observable x-ray variability is thus possible and is indeed seen. The variability of x-ray emission from x-ray active galaxies is described using, in the main, SSI observations. Some suggestive similarities in their light curves are noted. An x-ray luminosity function for Seyfert galaxies is constructed by two methods whose results agree well. Using this luminosity function the Seyfert galaxies are found to contribute = 6% to the diffuse x-ray background, without evolution. They can account for the entire background if only moderate evolution is allowed. Finally, recent developments in the field are noted: the discovery of more Seyfert galaxies as x-ray sources, variability, and some first x-ray spectra are reviewed. An extension to the class of 'x-ray active galaxies' in the form of high excitation emission line galaxies is reported. The great possibilities for future research in the area of x-ray active galaxies provided by new x-ray observatories in flight or under construction are explored briefly. It is clear that this is a subject just begun.