X-ray Diagnostics of Relativistic Reflection Around Supermassive Black Holes
Author(s)
Bhayani, Shyam
Type
Thesis
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.
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.
Date Issued
2011
Date Awarded
2011-10
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Nandra, Kirpal
Creator
Bhayani, Shyam
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)