Andrea Merloni
Queens' College, Cambridge
and
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.
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.
Please note that the thesis is formatted for A4 paper.