The X-ray spectra of active galaxies are, to a first approximation, power-law-like. Upon more detailed investigation, however, features become apparent: in particular, an emission line corresponding to iron, around 6.4 keV, and the so-called 'soft-excess' below ~ 2 keV, whereby an extrapolation of a power-law fit to the 2-10 keV energy band under-predicts the observed counts. In this thesis, the soft X-ray excesses in samples of both low- and high-luminosity Active Galactic Nuclei (Seyfert galaxies and QSOs respectively) are analysed. This allows the parameters of the soft excess, such as strength and temperature, to be compared with X-ray luminosity. It is found, however, that there are very few differences between Seyfert galaxies and QSOs, leading to the idea that the soft excess is not strongly linked to the luminosity of the object. Time variability of the soft excess is studied, by investigating three objects (3C 273, PKS 0558-504 and 1H 0419-577) which have each been observed a number of times in the X-ray band. The soft excess is found to vary in each, with the changes possibly due to the Compton cooling of the corona above the accretion disc, brought on by an increase in accretion rate. Another sample, of high-redshift objects, is also discussed, finding that there is no evidence for spectral evolution or intrinsic absorption in these radio-quiet QSOs. When considering this sample as a whole, there is an indication of spectral flattening at higher energies, suggesting the presence of a Compton reflection hump. Finally, the existence and strength of the neutral, narrow iron line is investigated, finding that, in the more luminous objects, the equivalent width is much lower, this is the X-ray equivalent of the Baldwin effect found in the optical and UV emission lines.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:400875 |
| Date | January 2003 |
| Creators | Page, Kim |
| Publisher | University of Leicester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2381/30671 |
Page generated in 0.0013 seconds