| Summary: | In this thesis I present a study of the properties of AGN populations based on X-ray surveys with ROSAT and Chandra. I have studied the short timescale X-ray variability properties of a sample of 156 radio-quiet quasars taken from the ROSAT PSPC archive covering a redshift range 0.1 < <i>z</i> < 4.1. I use a maximum likelihood method to constrain the amplitude of variability, and combine light curves in ensembles in order to identify correlations with luminosity and with redshift. Variability amplitude is found to be anti-correlated with luminosity for quasars out to redshifts of ~ 2, in good agreement with the relation found for local AGN (<i>z</i> < 0.1). There is tentative evidence for an increase in X-ray variability amplitude towards high redshifts (<i>z</i> > 2) in the sense that quasars of the same X-ray luminosity are more variable at <i>z</i> > 2. I have discussed possible explanations for this effect. The simplest explanation may be that high redshift quasars are accreting more efficiently than local AGN. The spectral shape of the hard X-ray background may be produced by the cumulative emission of a large population of obscured AGN. Comastri <i>et al. </i>(1995) have predicted the ratios of objects with X-ray column densities necessary to provide a fit to the X-ray background. Using these ratios together with the observed broad band spectral energy distributions of quasars, I have estimated the characteristics such a population would exhibit from submillimetre to ultraviolet wavelengths. Predictions for the contribution of these obscured AGN to mid-infrared surveys are a good fit to the observed number counts of sources displaying both 15<i>μ</i>m and X-ray emission. Finally, I have analysed X-ray source properties and statistics in two deep Chandra observations of the ELAIS fields N1 and N2. I present a comprehensive source catalogue of 233 point sources. In addition, 2 extended sources are detected and found to be associated with galaxy clusters. An overdensity of sources is found in regions N1 with 30% more sources than N2. Number count relations reveal a greater fraction of hard spectrum sources towards fainter X-ray fluxes. The fraction of sources with galaxy-like optical counterparts similarly increases towards faint fluxes. A possible explanation is that obscuration is causing intrinsically bright, softer-spectrum AGN to appear as faint, hard X-ray sources with galaxy-like optical counterparts.
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