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X-ray Diagnostics of Relativistic Reflection Around Supermassive Black Holes

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Title: X-ray Diagnostics of Relativistic Reflection Around Supermassive Black Holes
Authors: Bhayani, Shyam
Item Type: Thesis or dissertation
Abstract: This thesis presents an investigation into the X-ray energy and variability spectra of Seyfert galaxies. Reflection of the X-ray continuum by the accretion disc in these Active Galactic Nuclei (AGN) is expected to produce an iron line and an associated reflection continuum. The iron line should be relativistically broadened as it is subject to the strong gravitational field of the central black hole. Often, such a line is never found either because it appears not to be present or if it is, the magnitude of the broadening is less than expected. These cases pose a problem for the current understanding of accretion onto the black hole in AGN. We analyse the XMM-Newton observations of such objects to determine if the relativistically broadened iron line only appears to be “missing” because the accretion disc is ionised and/or there is a strong concentration of the line emission in the central disc regions. In the latter scenario, it is possible for the iron line to be smeared to such an extent that it blends into the continuum and is therefore difficult to disentangle. A combination of both effects leads to the identification of a relativistically broadened iron line in the majority of the observations where it could not be detected previously. In some of these objects, blurred disc reflection can also be detected if there is an additional source of iron line emission that arises further from the black hole such as in the Broad Line Region. We find that the strength of the blurred iron line and associated reflection continuum in these objects is consistent with the sources that exhibit a “classical” relativistically broadened iron line. Following this, we investigate the variability of the iron line in our sample of Seyferts. In models of accretion disc reflection, a simple prediction is that variations in the reflection component should follow variations in the X-ray continuum that illuminates the disc. Using unnormalised rms variance-spectra, we investigate if this scenario is typical in our sample of observations and find that it is not generally confirmed. Instead, there is a large amount of diversity in the behaviour of the iron line variability. For example, some observations possess an iron line that is more variable than the continuum, while other iron lines exhibit variability only bluewards of the line core. Most interestingly, a large group of observations possess a broad iron line that is invariant to the continuum and there are changes in the variability of the iron line between multiple observations of the same object. In some cases, the rms spectra can distinguish between models that explain the broad red wing of the iron line. Finally, we expand the variability analysis to between 0.3 – 10 keV and focus on the broadband X-ray variability on short time-scales (~ 40 ks). After normalising the variance spectra by the X-ray flux, a diverse range of amplitudes and shapes are found in the variance-spectra of both different objects and the same object. Part of the scatter in the amplitudes is due to the anti-correlation between the black hole mass of an AGN and its variance, while the various classes of profile shapes suggest that there are intrinsic differences in the spectral variability throughout the sample, as found with the iron line. We model variance-spectra that are based on relativistic disc reflection and velocity-smeared models of the broadband X-ray spectra of AGN and find that a range of profile shapes are possible. The mean variance-spectrum of those that are humped in the sample can be explained solely by a small increase in the slope of the continuum as the X-ray flux increases in a reflection-based model. Interestingly, there is no requirement for either the disc reflection or absorbers local to the AGN to respond to the changing illuminating intensity.
Issue Date: 2011
Date Awarded: Oct-2011
URI: http://hdl.handle.net/10044/1/8957
DOI: https://doi.org/10.25560/8957
Supervisor: Nandra, Kirpal
Author: Bhayani, Shyam
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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