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Title: Ionized accretion discs around black holes
Author: Ballantyne, D. R.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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Many galaxies in the Universe show evidence of a supermassive black hole at their dynamical centre. About 10-20% of these galaxies also contain an extremely bright, point-like continuum source in their nuclear region. These luminous objects are called active galactic nuclei (AGN), and are thought to be powered by material accreting onto the central black hole. The infalling gas loses its energy and angular momentum by passing through an accretion disc. The disc then radiates away this energy, and it is this radiation that is observed as an AGN. When observed in the X-ray waveband AGN are found to be very bright and rapidly variable (on timescales as small as 1000 s), implying that the emission must originate from the innermost regions of the central engine. X-ray spectra of AGN often exhibit distinct features that are attributed to radiation reflecting off of the accretion disc. Therefore, it is possible to use sensitive X-ray spectroscopic observations of AGN to directly probe the physics of accretion flows only a short distance away from the black hole. Comparing the results from AGN with different properties, such as luminosity or radio power, may allow general conclusions on the AGN phenomenon to be drawn. This thesis applies computations of X-ray reflection spectra to observations of different AGN in order to determine various properties of their accretion flows. The calculations take into account the ionization effects of the incident radiation and consider different density structures for the surface of the accretion disc. It was found that the model spectra were a good description of the observed X-ray data for many AGN between 1 and 10 keV. In particular, five narrow-line Seyfert 1 galaxies were well fit with reflection spectra from a highly ionized accretion disc, consistent with the idea that they contain rapidly accreting black holes. A similar result was found when fitting the data of 3C 120, a broad-line radio galaxy, which argues against the claim that radio-loud AGN have truncated accretion discs. On the other hand, a weakly ionized reflector proved to be a better fit to the Seyfert 1 galaxy MCG-6-30-15.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available