X-ray Variability, Excess Variances and the Black Hole Properties of AGN

Active galactic nuclei areamongthemostpersistently energetic objects observed in the known universe. Significant X-ray variability is observed in a large subset of sources, on both short (ks) and long ( days) timescales. Observations have demonstrated that all objects appear to vary differently and variability could provide an insight to the structure of active galactic nuclei. In this thesis studies into the variability of different samples of active galactic nuclei are presented to better understand how an object’s variability links to other measured quantities. In this thesis our current understanding of active galactic nuclei is first summarized, followed by a technical chapter outlining the missions from which data was collected and the basic analysis methods that were used in general. In chapter 3 X-ray data was used to create a serendipitous sample of active galactic nuclei candidates, which were matched with optical counterparts and filtered before their excess variances were calculated. Following on from this the excess variances of the AGN sample were compared against their luminosity and redshift, finding an anti-correlation. The results were also examined for the impact of time on the excess variance. In chapter 5 a sample of well observed active galactic nuclei was used to create and model their power spectral density, a different measure of X-ray variability. This was then observed to correlate to the mass of the object. Using these observations the mass of the previous sample of objects was estimated using a calculated relation between excess variance and mass. In the final science chapter a study into the predicted response of the recent Astrosat mission is carried out, alongside a comparison to its in-flight performance and howfuture Astrosat data could be used in continuing the previous studies.