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Title: Spectral signatures in the X-ray emission from active galactic nuclei.
Authors: Nandra, Kirpal.
Award date: 1991
Presented at: University of Leicester
Abstract: Spectral analysis of Ginga observations of Seyfert galaxies suggests a departure from the simple power law model which adequately described the HEAO-1 and EXOSAT data for these sources. In particular, compelling evidence is found for an Fe K-emission line, a K-absorption edge and a spectral flattening above about 10 keV. Analysis of a sample of AGN spectra shows that these features are extremely common. The mean energy of the emission feature suggests an origin in Fe K fluorescence from material in a low state of ionization, with a mean equivalent width ~ 150 eV. In contrast, highly ionized gas is required to reproduce the absorption edge energy. Monte Carlo modelling, carried out in various geometries, suggest that the line arises via fluorescence in cold, optically thick material out of the line of sight to the X-ray source, which is accompanied by continuum photons 'reflected' from the material. The characteristic shape of the reflected spectrum, when combined with the primary power law, leads to the observed emission line and 'hard tail'. The reflecting material must subtend a large solid angle at the X-ray source to explain the strength of the emission line and hard tail. Such a geometry is suggestive of an accretion disc. Observations of spectral variability in two Seyfert galaxies, MCG-6-30-15 and NGC 5548, show that such variability can be explained by varying proportions of directly viewed and reflected continua, constraining the extent of the reflecting material. Detailed analysis of the spectra of these two objects show that they are consistent with X-ray reflection from an accretion disc, with additional absorption by a highly ionized 'warm absorber' in the line of sight, as found for the bulk of the sample sources. In the case of NGC 5548, an extra component to the line emission from the warm absorber is also inferred. Concluding, the accretion disc reflection model provides an excellent description of the observed spectral features and variability in Seyfert galaxies, but a complex of emission lines around 6.5 keV seems highly likely. Future observations of these objects, with improved spectral resolution, should therefore be of great value.
Level: Doctoral
Qualification: Ph.D.
Rights: Copyright © the author. All rights reserved.
Appears in Collections:Theses, Dept. of Physics and Astronomy
Leicester Theses

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