The United Nations and the problems of Palestine 1947-1949

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Obscuration and X-ray variability of active galactic nuclei

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 2) in the sense that quasars of the same X-ray luminosity are more variable at z > 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 et al. (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/im and X-ray emissionFinally, I have analysed X-ray source properties and statistics in two deep Chandra observations of the ELAIS fields N1 & 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 region 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