16,242 research outputs found
Bayesian Model Selection with Future 21cm Observations of The Epoch of Reionisation
We apply Bayesian statistics to perform model selection on different
reionisation scenarios via the Multinest algorithm. Initially, we recover the
results shown by 21CMMC for the parameter estimation of 21cmFAST models. We
proceed to test several toy models of the Epoch of Reionisation (EoR) defined
in contrasting morphology and scale. We find that LOFAR observations are
unlikely to allow model selection even with long integration times. HERA would
require 61 dipoles to perform the same analysis in 1080 hours, and becomes
comparable to the SKA with 217 dipoles. We find the SKA requires only 324 hours
of observation to conclusively distinguish between our models. Once model
selection is achievable, an analysis of observational priors is performed
finding that neutral fraction checks at specific redshifts add little to no
inference. We show the difficulties in model selection at the level of
distinguishing fiducial parameters within a model or distinguishing galaxies
with a constant versus power law mass-to-light ratio. Finally, we explore the
use of the Savage-Dickey density ratio to show the redundancy of the parameter
Rmfp within 21cmFAST.Comment: 20 pages, 16 figures - Accepted for publication by MNRA
Forecasted 21 cm constraints on compensated isocurvature perturbations
A "compensated" isocurvature perturbation consists of an overdensity (or
underdensity) in the cold dark matter which is completely cancelled out by a
corresponding underdensity (or overdensity) in the baryons. Such a
configuration may be generated by a curvaton model of inflation if the cold
dark matter is created before curvaton decay and the baryon number is created
by the curvaton decay (or vice-versa). Compensated isocurvature perturbations,
at the level producible by the curvaton model, have no observable effect on
cosmic microwave background anisotropies or on galaxy surveys. They can be
detected through their effect on the distribution of neutral hydrogen between
redshifts 30 to 300 using 21 cm absorption observations. However, to obtain a
good signal to noise ratio, very large observing arrays are needed. We estimate
that a fast Fourier transform telescope would need a total collecting area of
about 20 square kilometers to detect a curvaton generated compensated
isocurvature perturbation at more than 5 sigma significance.Comment: 7 pages, v2: minor typos corrected, reflects PRD accepted versio
Reduction of nitric oxide emissions from a combustor
A turbojet combustor and method for controlling nitric oxide emissions by employing successive combustion zones is described. After combustion of an initial portion of the fuel in a primary combustion zone, the combustion products of the primary zone are combined with the remaining portion of fuel and additional plenum air and burned in a secondary combustion zone under conditions that result in low nitric oxide emissions. Low nitric oxide emissions are achieved by a novel turbojet combustor arrangement which provides flame stability by allowing stable combustion to be accompanied by low nitric oxide emissions resulting from controlled fuel-lean combustion (ignited by the emission products from the primary zone) in a secondary combustion zone at a lower combustion temperature resulting in low emission of nitric oxide
Distinguishing models of reionization using future radio observations of 21-cm 1-point statistics
We explore the impact of reionization topology on 21-cm statistics. Four
reionization models are presented which emulate large ionized bubbles around
over-dense regions (21CMFAST/ global-inside- out), small ionized bubbles in
over-dense regions (local-inside-out), large ionized bubbles around under-dense
regions (global-outside-in) and small ionized bubbles around under-dense
regions (local-outside-in). We show that first-generation instruments might
struggle to distinguish global models using the shape of the power spectrum
alone. All instruments considered are capable of breaking this degeneracy with
the variance, which is higher in outside-in models. Global models can also be
distinguished at small scales from a boost in the power spectrum from a
positive correlation between the density and neutral-fraction fields in
outside-in models. Negative skewness is found to be unique to inside-out models
and we find that pre-SKA instruments could detect this feature in maps smoothed
to reduce noise errors. The early, mid and late phases of reionization imprint
signatures in the brightness-temperature moments, we examine their model
dependence and find pre-SKA instruments capable of exploiting these timing
constraints in smoothed maps. The dimensional skewness is introduced and is
shown to have stronger signatures of the early and mid-phase timing if the
inside-out scenario is correct.Comment: 18 pages, 13 figures, updated to agree with published versio
The impact of spin temperature fluctuations on the 21-cm moments
This paper considers the impact of Lyman-alpha coupling and X-ray heating on
the 21-cm brightness-temperature one-point statistics (as predicted by
semi-numerical simulations). The X-ray production efficiency is varied over
four orders of magnitude and the hardness of the X-ray spectrum is varied from
that predicted for high-mass X-ray binaries, to the softer spectrum expected
from the hot inter-stellar medium. We find peaks in the redshift evolution of
both the variance and skewness associated with the efficiency of X-ray
production. The amplitude of the variance is also sensitive to the hardness of
the X-ray SED. We find that the relative timing of the coupling and heating
phases can be inferred from the redshift extent of a plateau that connects a
peak in the variance's evolution associated with Lyman-alpha coupling to the
heating peak. Importantly, we find that late X-ray heating would seriously
hamper our ability to constrain reionization with the variance. Late X-ray
heating also qualitatively alters the evolution of the skewness, providing a
clean way to constrain such models. If foregrounds can be removed, we find that
LOFAR, MWA and PAPER could constrain reionization and late X-ray heating models
with the variance. We find that HERA and SKA (phase 1) will be able to
constrain both reionization and heating by measuring the variance using
foreground-avoidance techniques. If foregrounds can be removed they will also
be able to constrain the nature of Lyman-alpha coupling.Comment: 16 pages, 13 figure, 1 table. Accepted for publication in MNRA
Gradient methods in control theory. Part 2 - Sequential gradient-restoration algorithm
Sequential gradient-restoration algorith
The Stokes boundary layer for a thixotropic or antithixotropic fluid
We present a mathematical investigation of the oscillatory boundary layer (‘Stokes layer’) in a semi-infinite fluid bounded by an oscillating wall (the socalled ‘Stokes problem’), when the fluid has a thixotropic or antithixotropic rheology. We obtain asymptotic solutions in the limit of small-amplitude oscillations, and we use numerical integration to validate the asymptotic solutions and to explore the behaviour of the system for larger-amplitude oscillations. The solutions that we obtain differ significantly from the classical solution for a Newtonian fluid. In particular, for antithixotropic fluids the velocity reaches zero at a finite distance from the wall, in contrast to the exponential decay for a thixotropic or a Newtonian fluid. For small amplitudes of oscillation, three regimes of behaviour are possible: the structure parameter may take values defined instantaneously by the shear rate, or by a long-term average; or it may behave hysteretically. The regime boundaries depend on the precise specification of structure build-up and breakdown rates in the rheological model, illustrating the subtleties of complex fluid models in non-rheometric settings. For larger amplitudes of oscillation the dominant behaviour is hysteretic. We discuss in particular the relationship between the shear stress and the shear rate at the oscillating wall
21-cm signatures of residual HI inside cosmic HII regions during reionization
We investigate the impact of sinks of ionizing radiation on the
reionization-era 21-cm signal, focusing on 1-point statistics. We consider
sinks in both the intergalactic medium and inside galaxies. At a fixed filling
factor of HII regions, sinks will have two main effects on the 21-cm
morphology: (i) as inhomogeneous absorbers of ionizing photons they result in
smaller and more widespread cosmic HII patches; and (ii) as reservoirs of
neutral gas they contribute a non-zero 21-cm signal in otherwise ionized
regions. Both effects damp the contrast between neutral and ionized patches
during reionization, making detection of the epoch of reionization with 21-cm
interferometry more challenging. Here we systematically investigate these
effects using the latest semi-numerical simulations. We find that sinks
dramatically suppress the peak in the redshift evolution of the variance,
corresponding to the midpoint of reionization. As previously predicted,
skewness changes sign at midpoint, but the fluctuations in the residual HI
suppress a late-time rise. Furthermore, large levels of residual HI
dramatically alter the evolution of the variance, skewness and power spectrum
from that seen at lower levels. In general, the evolution of the large-scale
modes provides a better, cleaner, higher signal-to-noise probe of reionization.Comment: Minor edits to agree with MNRAS published versio
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