M. J. Page
X-ray evolution and variability of active galactic nuclei
1997 (Supervisor: K. O. Mason)

The Luminosity function and cosmic evolution of active galactic nuclei (AGN) selected in the 0.5 - 2 keV X-ray band is studied using the sample of AGN from the RIXOS survey. It is found that pure luminosity evolution models represent the data well, provided rapid evolution stops at redshift (~2). The evolution of narrow emission line galaxies (NELGs), which may be important contributors to the cosmic X-ray background, is examined using the sample of NELGs obtained from the ROSAT UK Deep Survey and RIXOS. I detect evolution at high significance but find that this evolution is probably slower than the evolution of broad line AGN, and/or ceases at a lower redshift. Data from deeper ROSAT surveys are used to extend the AGN luminosity function to lower luminosities and higher redshifts, and biases caused by the dispersion of AGN spectral slopes are incorporated in the analysis. Some deviations from pure luminosity evolution are seen for a critical (q0=0.5) universe, and the previous conclusions regarding the lack of evolution at high redshift are strengthened. The possibility of improved surveys of AGN with the next generation of X-ray satellites is investigated. An efficient survey for the study of the high redshift (z>2) AGN luminosity function is described. The X-ray variability of the narrow line Seyfert 1 galaxy Markarian 766 is studied using ROSAT data. The spectrum is well described by a power law and a blackbody soft excess. The power law component varies continuously but variability of the soft excess is not detected within the observations. The power law component is always steeper when it is brighter. This variability can be explained if the power law is produced by thermal or non-thermal Comptonisation of soft photons. The behaviour of Markarian 766 is analogous to that of Galactic black hole candidates in the low state. Finally, the results of this thesis are related to the evolution and history of individual active galaxies.