SED & Variability Studies of AGN

Abstract

This thesis contains the results of studies into the causes of variability seen in the optical and X-ray emission from matter accreting onto supermassive black holes (SMBHs) in active galactic nuclei (AGN).

The first chapter provides a brief overview of the history of optical and X-ray astronomy along with a summary of the principle properties of AGN. The second chapter investigates the dust-to-gas relationship for a sample of 1,201 X-ray detected quasars. We carry out an in-depth optical and X-ray spectral analysis and produce composite spectra for gas absorbed and dust reddened quasars. We find that the objects with the largest gas column densities are not the objects with the largest amounts of dust reddening. This study also highlights problems in constraining absorption levels in objects with low-quality X-ray spectra.

We then present optical-to-X-ray spectral energy distributions (SEDs) for 170 Type 1 AGN. We use a new broadband SED model, which combines the standard disc emission with low- and high- temperature Comptonisation components through the introduction of a coronal radius. We find that objects that have an Eddington ratio greater than unity have larger disc luminosities, lower black hole mass, higher mass accretion rate and smaller coronal radii, than the rest of the sample.

We conduct a detailed optical and X-ray variability study on a subset of 41 of these objects in the fourth chapter. We combined optical light curves from the Catalina Sky Survey (CSS) with long duration X-ray observations from the XMM-Newton satellite. We found that the objects with the lowest black hole masses were the least variable. However, we did not find the expected relationship between luminosity and variability, probably due to the wavelength range observed by CSS and selection biases in our sample.

The final two chapters report results from pilot studies based on data from the Pan-STARRS telescope. In the fifth chapter we present an in-depth study of a highly variable object discovered as part of the Pan-STARRS supernovae search, and in the sixth chapter we investigate a subset of the light curves produced in the Medium Deep Survey (MDS) by analysing the light curves of 39 quasars including 12 broad absorption line quasars (BALQSOs). Finally, we also examine the potential for future studies, by cross-matching a large sample of spectroscopically identified Seyfert 2 AGN to the large area 3π catalogue.