Properties of soft X-ray bright Active Galactic Nuclei

Studies of the extremes of behaviour observed in Active Galactic Nuclei (AGN) constrain the physics of their emission processes by pushing existing models to their limits. Narrow-Line Seyfert 1 (NLS1s) represent one extreme of the AGN phenomenon, and soft X-ray selection is an efficient way to find NLS1s. This thesis examines the X-ray and optical properties of soft X-ray bright AGN and NLS1s in particular. The X-ray spectral properties, as observed by ASCA , of a sample of 22 NLS1s are discussed. The X-ray continua show a wide range in slope, with a mean only slightly steeper than that of 'normal' Seyfert Is and most show 'soft excess' emission, often containing a significant fraction of the X-ray luminosity. In addition, some NLS1s show spectral features in the range 0.7-0.9 keV and 1.1-1.4 keV, which are described in terms of absorption in photoionised material along the line-of-sight. The X-ray properties of the bright NLS1 Ark 564 are examined in detail and the X-ray spectrum is found to be consistent with a model comprising a power-law plus 'reflection' from ionised matter. A re-analysis of the ROSAT Wide Field Camera all-sky survey data is presented. A total of 19 AGN are detected in the extreme-ultraviolet, including eight NLS1s, making this the first complete sample to contain a high fraction of NLS1s. This work is extended using a larger, complete sample of AGN selected on the basis of 0.25 keV flux. Correlations between the measured X-ray and optical properties are discussed and the sample is used to isolate objects at the extreme ends of the parameter space explored. The luminosity function of the sample is presented, along with the luminosity functions of Seyferts classified on the basis of H/3 width and X-ray spectral slope.