High energy studies of active galactic nuclei with XMM-Newton.

X-ray astronomers currently have access to two major X-ray observatories, XMM-Newton and Chandra. While targeted observations are likely to continue for the next few years, this will not be the case forever. Archival data will thus become an increasingly important resource. X-ray observations produce large samples of active galaxies with fewer biases than at other wavelengths. In this thesis, techniques for archival survey work are developed and applied to small 'pilot' samples of AGN from the XMM-Newton archive. A photometric redshift method for use with X-ray selected AGN is developed and tested with both faint and bright samples (from the Lockman Hole region and 2XMM serendipitous catalogue respectively). A level of accuracy surpassing other X-ray photometric redshift methods is found. A deep look at the 3C 273 field from multiple XMM-Newton observations is used to investigate continuum X-ray variability in serendipitous AGN in the field. The photometric redshift method developed is used to determine X-ray luminosities for the serendipitous sources and the commonly found anti-correlation between excess variance and X-ray luminosity is reproduced. The connection between continuum variability in the X-ray and optical/UV bands is investigated using a sample of Seyfert 1 galaxies from the XMM-Newton Science Archive. Cross correlation analysis of the light curves on timescales of ≈1000s finds no significant evidence for correlation between the bands. A larger sample of sources is taken from the serendipitous source catalogue 2XMMp to investigate cosmic variance and bias of X-ray selected sources. For full band (0.2 - 12 keV) sources above a flux limit of l x 10-14 erg cm-2 S-1, AGN bias, which characterises how AGN mass traces the underlying mass, is found to be in agreement (within the error bounds) with the bias factor determined from much larger optical surveys.