Physics, School of
https://hdl.handle.net/1842/119
2020-02-18T03:51:03ZBroad emission line variability in active galactic nuclei on long timescales
https://hdl.handle.net/1842/36747
Broad emission line variability in active galactic nuclei on long timescales
Homan, David Sebastiaan
Active Galactic Nuclei are among the most powerful sources of radiation in the Universe and
are known to show variability across the EM spectrum. This thesis will focus on the variability
in the optical and UV range, which is dominated by emission from the central accretion disc
and the Broad Line Region (BLR). All AGN are powered by the accretion of matter onto a
supermassive black hole. The BLR reprocesses part of the continuum generated by the accretion disc, re-emitting the energy in atomic emission lines. Variations in the continuum lead to
variations in the broad emission lines, however the manner in which the broad lines respond
is complex. Neither the accretion disc nor the BLR are currently fully understood. Over the
past years the availability of large data-sets from long term spectroscopic observations, such as
the Sloan Digital Sky Survey (SDSS), has greatly improved our ability to study the variability
of AGN, expanding the observed timescales to decades. These timescales match the dynamical
timescales associated with the BLR, implying that we could be able to track structural changes
in this region.
The research presented in this thesis will use the response of the broad lines to the continuum to investigate the properties of the BLR, with an emphasis on long timescales. The
research is split over two independent, but closely linked studies. The first is a case study of
the highly variable active galaxy Markarian 110. The second study focusses on one particular
emission line: MgIIλ2798. Both studies rely on optical spectroscopic data. A pipeline was
written to fit archival and new spectra included in the study.
Mrk 110 is a local Seyfert I, at z=0.035, with available spectroscopic data going back
decades. Combining archival data with new observations from the William Herschel and Tillinghast Telescopes, it is possible to track Mrk 110 through dips and peaks in the continuum
at a relatively high cadence. Of particular note is the behaviour of HeIIλ4686, which varies
significantly more than the other emission lines. This line can be used as a proxy for the ionising
(FUV) part of the continuum. Comparing Hα, Hβ, and HeIλ5876 with the FUV flux we note
that the response of the line fluxes to continuum changes evolves considerably on the timescale
of years. For the highest continuum fluxes it is possible to detect a saturation level of the line
responses, correlated with the Reverberation Mapping lags associated with the lines. Analysing
the line profiles, we can detect an offset between the narrow and broad components of the Hβ
and HeIIλ4686 lines, likely associated with a radial flow in the BLR.
The Magnesium line at 2798Å
is a prominent feature in most AGN spectra. Its response to
continuum changes has been shown to be limited in previous studies. The study presented here
will make use of two data-sets. The first is a sample of 43 extremely variable quasars. This
sample combines SDSS spectra with new observations from the William Herschel, Magellan,
and MTT Telescopes. The second data-set consists of approximately 16,000 SDSS quasars with
repeat spectroscopy. The data from both samples indicate that there is in fact a broad range
in MgII behaviour. We observe that the line tracks the continuum on average, but also note
considerable scatter in this correlation.
Combining the results from the Mrk 110 and MgII studies we find that there is evidence
in all samples for broad line responsivity that evolves over time. There is also considerable
evidence to suggest that the dynamics of both the HeIIλ4686 and the MgII line are not only
set by their positions in the gravitational potential.
2019-12-23T00:00:00ZLattice QCD calculation of beyond the standard model kaon mixing at the physical point
https://hdl.handle.net/1842/36746
Lattice QCD calculation of beyond the standard model kaon mixing at the physical point
Kettle, Julia Rachael
This thesis presents beyond the standard (BSM) model kaon mixing results
calculated on the lattice with simulations at the physical light quark mass. These
results were calculated using RBC-UKQCD 2+1 flavour domain wall fermion
simulations at the iso-spin symmetric limit, with pion masses ranging from the
physical value up to 430 MeV. This thesis presents the bare results of BSM kaon
mixing bag parameters and ratios, alongside strange and light meson masses
and decay constants, it then details the renormalisation procedure and chiral
continuum extrapolation. Finally the renormalised continuum physical point
results of beyond the standard model bag parameters, ratio parameters and
matrix elements and the standard model bag parameter are presented. The
beyond the standard model kaon mixing matrix elements will improve upon
previous calculations' precision, and these results will help address previous
tensions seen in some of the BSM matrix elements. These quantities are important
in the search for new physics.
2019-12-23T00:00:00ZCrystal structures and phase transitions of bismuth-tellurides under high pressure
https://hdl.handle.net/1842/36745
Crystal structures and phase transitions of bismuth-tellurides under high pressure
Freeman, Kenneth Nicol
Bismuth-tellurides are binary compounds of bismuth (Bi) and tellurium (Te)
which form an infinitely adaptive series, (Bi2)m(Bi2Te3)n, within certain compositional
limits. Members of this series exhibit a number of interesting and
useful physical properties: they are among the most widely known thermoelectric
materials and they have been shown to exhibit superconductivity and topological
insulation. Many of these properties have been reported to be induced or
enhanced by the application of high pressure. Despite this, the current
understanding of the crystal structures of the (Bi2)m(Bi2Te3)n series under high
pressure remains fragmentary. Knowledge of these crystal structures is the first
necessary step towards further investigation of the properties of these materials
through, for example, electronic structure calculations.
This thesis presents the analysis of high-pressure x-ray diffraction data collected
for several members of the (Bi2)m(Bi2Te3)n series. Angle-dispersive x-ray
powder diffraction experiments were performed at synchrotron facilities, utilising
diamond-anvil pressure cells to generate pressures up to 26 GPa. Several
structural phases of the (Bi2)m(Bi2Te3)n series are investigated in detail including
a previously unreported complex host-guest structure that forms in several of the
series members at high pressures. This is a similar structure to the host-guest
phase of elemental bismuth (Bi-III) with a host framework enclosing linear guest
chains which lie along the c-axis direction. The guest chains are found to be
disordered along their lengths, contributing only diffuse features to the measured
x-ray powder diffraction profiles. Along with this structural disorder, this
structure is found to include chemical ordering with the guest chains composed
primarily of bismuth
Other investigated phases include the layered structure found at ambient
conditions, consisting of regular Bi2 and Bi2Te3 blocks stacked along the c-axis.
Where appropriate, Rietveld refinement of these structures found the block
compositions to differ from the idealised structure. For certain compositions, a
four-dimensional modulated structure is the more appropriate description; le Bail
fits were performed in these cases to provide the pressure-evolution of the lattice
parameters and modulation vector.
All investigated samples were found to adopt high-symmetry cubic phases at
the highest pressures investigated here. This, along with various similarities
between the other pressure-induced phases, suggests a universal behaviour in
the (Bi2)m(Bi2Te3)n series on pressurisation. Composition was found to have a
significant influence on the behaviour under pressure and individual structures
of bismuth-tellurides. This work represents the first systematic, high-pressure
structural study of these materials and explores the pressure-behaviour of the
series as a whole and as a function of composition. This provides a first necessary
step towards an improved understanding of these materials and their properties
at high pressures.
2019-12-23T00:00:00ZNonequilibrium steady states from a random-walk perspective
https://hdl.handle.net/1842/36698
Nonequilibrium steady states from a random-walk perspective
Wood, Anthony James
It is well known that at thermal equilibrium (whereby a system has settled into a steady
state with no energy or mass being exchanged with the environment), the microstates of
a system are exponentially weighted by their energies, giving a Boltzmann distribution.
All macroscopic quantities, such as the free energy and entropy, can be in principle
computed given knowledge of the partition function. In a nonequilibrium steady state,
on the other hand, the system has settled into a stationary state, but some currents
of heat or mass persist. In the presence of these currents, there is no unified approach
to solve for the microstate distribution. This motivates the central theme of this work,
where I frame and solve problems in nonequilibrium statistical physics in terms of
random walk and diffusion problems.
The system that is the focus of Chapters 2, 3, and 4 is the (Totally) Asymmetric
Simple Exclusion Process, or (T)ASEP. This is a system of hard-core particles making
jumps through an open, one-dimensional lattice. This is a paradigmatic example
of a nonequilibrium steady state that exhibits phase transitions. Furthermore, the
probability of an arbitrary configuration of particles is exactly calculable, by a matrix
product formalism that lends a natural association between the ASEP and a family of
random walk problems.
In Chapter 2 I present a unified description of the various combinatorial interpretations
and mappings of steady-state configurations of the ASEP. As well as deriving new
results, I bring together and unify results and observations that have otherwise been
scattered in the combinatorics and physics literature. I show that particular particle
configurations of the ASEP have a one-to-many mapping to a set of more abstract
paths, which themselves have a one-to-many mapping to permutations of numbers.
One observation from this wider literature has been that this mapped space can
be interpreted as a larger set of configurations in some equilibrium system. This
naturally gives an interpretation of ASEP configuration probabilities as summations
of Boltzmann weights. The nonequilibrium partition function of the ASEP is then a
summation over this equilibrium ensemble, however one encounters difficulties when
calculating more detailed measures of this state space, such as the entropy.
This motivates the work in Chapter 3. I calculate a quantity known as the Rényi
entropy, which is a measure of the partitioning of the state space, and a deformation of
the familiar Shannon entropy. The Rényi entropy is simple for an equilibrium system,
but has yet to be explored in a classical nonequilibrium steady state. I use insights
from Chapter 2 to frame one of these Rényi entropies | requiring the enumeration
of the squares of configuration weights | in terms of a two-dimensional random walk
with absorbing boundaries. I find the appropriate generating function across the full
phase diagram of the TASEP by generalising a mathematical technique known as
the obstinate kernel method. Importantly, this nonequilibrium Rényi entropy has a
different structural form to any equilibrium system, highlighting a clear distinction
between equilibrium and nonequilibrium distributions.
In Chapter 4 I continue to examine the Rényi entropy of the TASEP, but now
performing a time and space continuum limit of the random walk problem in Chapter 3.
The resultant problem is a two-dimensional dffusion problem with absorbing boundary
conditions, which once solved should recover TASEP dynamics about the point in the
phase diagram where the three dynamical phases meet. I derive a generating function,
sufficiently simple that its singularities can be analysed by hand. This calculation
entails a novel generalisation of the obstinate kernel method of Chapter 3: I find a
solution by exploiting a symmetry in the Laplace transform of the diffusion equation.
I finish in Chapter 5 by introducing and solving another nonequilibrium system, termed
the many-filament Brownian ratchet. This comprises an arbitrary number of filaments
that stochastically grow and contract, with the net effect of moving a drift-diffusing
membrane by purely from thermal fluctuations and steric interactions. These dynamics
draw parallels with those of actin filament networks at the leading edge of eukaryotic
cells, and this improves on previous 'pure ratchet' models by introducing interactions
and heterogeneity in the filaments. I find an N-dimensional diffusion equation for the
evolution of the N filament-membrane displacements. Several parameters can be varied
in this system: the drift and diffusion rates of each of the filaments and membrane, the
strength of a quadratic interaction between each filament with the membrane, and the
strength of a surface tension across the filaments. For several interesting physical cases
I find the steady-state distribution exactly, and calculate how the mean velocity of the
membrane varies as a function of these parameters.
2019-12-16T00:00:00Z