The spectra and variability of accreting black holes

Andrea Merloni

Queens' College, Cambridge
Institute of Astronomy

A dissertation submitted for the degree of
Doctor of Philosophy in the University of Cambridge

2 October 2001

See below to download the thesis


The spectral energy distribution (SED) of the gravitational power released as electromagnetic radiation when matter accretes onto
a compact object and its temporal variability is the subject of my dissertation. Such SED is far from universal. Different accretion modes are possible, with different radiative properties. The main goal of accretion flows theory is to distinguish and understand all the possible
different modes of accretion, and classify the observed sources in terms of of such modes. The state of the art of both accretion theory and observations is briefly reviewed in the first chapter. The spectra of accreting black holes clearly show different components, that
are produced by different physical processes and originate from different regions of the accretion flow. One of those components is a quasi-thermal hump, produced by a geometrically thin,  optically thick accretion disc, and its spectrum is discussed in the fourth chapter,
where I also present a critical analysis of the usual interpretation of the most common model used to fit observed disc spectra (the so-called multicolour disc model).  I show that such a  model not only underestimates systematically the value of the inner disc radius,  but that, when the accretion rate through the disc is allowed to change, the inner edge of the  disc, as inferred from the multicolour model, appears to
move even when it is in fact fixed  at the innermost stable orbit. Most accreting black holes,  either of stellar mass or supermassive, when observed with hard X-rays show also signs of a hot Comptonizing component in the flow,  the so-called corona. This is the main subject of the work presented in the second and third  chapters of the dissertation. Firstly, I argue that the observed properties of hard X-ray emission from accreting black holes constitute  strong supporting evidence for a magnetically-dominated corona. I demonstrate how the  inferred  thermal energy content of the corona is far too low to explain the observed X-ray luminosities, unless  the corona itself is in fact a reservoir, where the energy is mainly  stored in the form of a magnetic field. Then I consider the emission  of galactic black holes in the low/hard state and of low-luminosity AGN, that all are thought to be sources accreting at low rates. Advection (or convection) dominated accretion flows
are usually considered the best candidates to explain their SED. I present an alternative possibility, involving strong, unbound, magnetic coronae. I discuss reasons why energetically dominant coronae are ideal sites for launching powerful jets/outflows, both MHD and thermally driven. In analysing the spectral properties of such coronal outflow dominated accretion discs, I reach the important conclusion that if the jet/outflow is, as it is likely, radiatively inefficient, then so is the source overall, even without advection of energy into the black hole
being relevant for the dynamics of the accretion flow. To conclude the part of the thesis dedicated to the spectral properties, in the third chapter I will describe the effects of the presence of a  hot corona on the observational properties of the reflection components. In the fifth chapter I consider the rapid aperiodic time variability observed in  accreting black holes, focusing in particular on the simultaneous spectral  variations associated with such noise. X-ray observations of Seyfert 1 galaxies offer  the unique possibility of observing spectral variability
on timescales comparable to the dynamical time of the inner accretion flow. I show how a model of the heating of the corona in terms of correlated reconnecting magnetic structures (the `thundercloud model') can be used to explain the observed short-term spectral and temporal
variability. Finally, in the sixth chapter I apply the magnetic corona models previously discussed  to the  X-ray transient XTE J1118+480, which displays spectral properties typical of GBHC in the low/hard state, and unusually strong  quasi-periodic oscillations both in the optical and in the X-ray bands. I make the case for a magnetically dominated outflowing corona as a plausible explanation
for all the above mentioned phenomena in this source.

Outline of thesis

Chapter 1
After a brief introduction, I review the main theoretical models for accretion flows around black holes, and discuss the observational state of the art.

Chapter 2
This chapter deals with the physical properties of the part of the accretion flow which is responsible for the hard X-ray power-law emission, ubiquitous in the spectra of black hole powered sources: the so-called corona. It will be demonstrated how the corona must be magnetically dominated  and therefore its physics related to the fundamental issue of the  nature of the magnetic viscosity inside the accretion disc itself.  Also in this chapter, a structured magnetic corona model for the primary high energy continuum of black hole spectra will be presented. Finally, I will discuss the relationships between coronal power and accretion rate and the role of the coronal medium in launching powerful outflows/jets from the inner regions of the accretion flow and propose an alternative scenario for the accretion mode in low-luminosity AGN and GBHC in their low/hard state.

Chapter 3
Here the effects of a Comptonizing corona on the appearance of the secondary spectral components (reflection features, fluorescent iron lines, etc.) will be discussed.

Chapter 4
To conclude the part of this dissertation that is focused on black hole spectral properties, the spectrum produced by an optically thick and geometrically thin accretion disc will be discussed in this chapter. It will be shown  that careful modeling of the disc vertical structure is necessary, for any meaningful conclusion about the inner disc geometry to be drawn from accretion disc spectroscopy alone.

Chapter 5
This chapter deals with the complementary problem of aperiodic time variability in the emission from accretion flows around black holes.
A model (the `thundercloud model') will be presented aimed at bridging X-ray spectral and temporal studies,  in order to put tighter constraints on the nature of the inner part of the accretion  disc--corona system. As an aside, we will also show how the study of accretion
flows in the time domain could shed some light on one of the biggest mysteries of high energy  astrophysics: gamma-ray bursts (GRB) and black hole formation.  We found a new correlation between the durations of subsequent emission episodes in the lightcurves of long, variable
bursts. Such a  correlation, analogous to that found in accreting black hole sources, may provide us with vital information, not only on the nature of the central engines of GRBs, but also on the very nature of the accretion flow in such extreme conditions as those found during the collapsing stages of a massive star.

Chapter 6
Here the model of a structured magnetic corona and of coronal  outflow dominated accretion discs is applied to the X-ray transient source XTE J1118+480. This is a key object for black hole astronomy, as its position high on the galactic plane during its last recent outburst has allowed us to determine its broadband spectrum with unprecedented accuracy. Once again, in the study of this source we have tried to take full advantage of simultaneous temporal and spectral analysis to constrain the emission properties and the elusive geometry of the inner accretion flow in GBHC in their low/hard state.

Chapter 7
is a summary of the most important ideas from each chapter. Ideas for future works are also discussed. 

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    Last modified  March 2002