21 research outputs found
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
Epoch of reionization parameter estimation with the 21-cm bispectrum
We present the first application of the isosceles bispectrum to MCMC
parameter inference from the cosmic 21-cm signal. We extend the MCMC sampler
21cmMC to use the fast bispectrum code, BiFFT, when computing the likelihood.
We create mock 1000h observations with SKA1-low, using PyObs21 to account for
uv-sampling and thermal noise. Assuming the spin temperature is much higher
than that of the CMB, we consider two different reionization histories for our
mock observations: fiducial and late-reionization. For both models we find that
bias on the inferred parameter means and 1- credible intervals can be
substantially reduced by using the isosceles bispectrum (calculated for a wide
range of scales and triangle shapes) together with the power spectrum (as
opposed to just using one of the statistics). We find that making the
simplifying assumption of a Gaussian likelihood with a diagonal covariance
matrix does not notably bias parameter constraints for the three-parameter
reionization model and basic instrumental effects considered here. This is true
even if we use extreme (unlikely) initial conditions which would be expected to
amplify biases. We also find that using the cosmic variance error calculated
with Monte-Carlo simulations using the fiducial model parameters whilst
assuming the late-reionization model for the simulated data also does not
strongly bias the inference. This implies we may be able to sparsely sample and
interpolate the cosmic variance error over the parameter space, substantially
reducing computational costs. All codes used in this work are
publicly-available.Comment: 12 pages, 11 figures (submitted to MNRAS
Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum
The epoch of reionization (EoR) 21-cm signal is expected to be highly
non-Gaussian in nature and this non-Gaussianity is also expected to evolve with
the progressing state of reionization. Therefore the signal will be correlated
between different Fourier modes (). The power spectrum will not be able
capture this correlation in the signal. We use a higher-order estimator -- the
bispectrum -- to quantify this evolving non-Gaussianity. We study the
bispectrum using an ensemble of simulated 21-cm signal and with a large variety
of triangles. We observe two competing sources driving the non-Gaussianity
in the signal: fluctuations in the neutral fraction () field and
fluctuations in the matter density field. We find that the non-Gaussian
contribution from these two sources vary, depending on the stage of
reionization and on which modes are being studied. We show that the sign of
the bispectrum works as a unique marker to identify which among these two
components is driving the non-Gaussianity. We propose that the sign change in
the bispectrum, when plotted as a function of triangle configuration
and at a certain stage of the EoR can be used as a confirmative
test for the detection of the 21-cm signal. We also propose a new consolidated
way to visualize the signal evolution (with evolving or
redshift), through the trajectories of the signal in a power spectrum and
equilateral bispectrum i.e. space.Comment: 18 pages, 11 figures. Accepted for publication in MNRAS. Replaced to
match the accepted versio
Gaussian process regression for foreground removal in hi intensity mapping experiments
We apply for the first time Gaussian Process Regression (GPR) as a foreground removal technique in the context of single-dish, low redshift H I intensity mapping, and present an open-source PYTHON toolkit for doing so. We use MeerKAT and SKA1-MID-like simulations of 21 cm foregrounds (including polarization leakage), H I cosmological signal, and instrumental noise. We find that it is possible to use GPR as a foreground removal technique in this context, and that it is better suited in some cases to recover the H I power spectrum than principal component analysis (PCA), especially on small scales
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The 21-cm bispectrum as a probe of non-Gaussianities due to X-ray heating
We present analysis of the normalized 21-cm bispectrum from fully-numerical simulations of intergalactic-medium heating by stellar sources and high-mass X-ray binaries (HMXBs) during the cosmic dawn. Ly-α coupling is assumed to be saturated, we therefore probe the nature of non-Gaussianities produced by X-ray heating processes. We find the evolution of the normalized bispectrum to be very different from that of the power spectrum. It exhibits a turnover whose peak moves from large to small scales with decreasing redshift, and corresponds to the typical separation of emission regions. This characteristic scale reduces as more and more regions move into emission with time. Ultimately, small-scale fluctuations within heated regions come to dominate the normalized bispectrum, which at the end of the simulation is almost entirely driven by fluctuations in the density field. To establish how generic the qualitative evolution of the normalized bispectrum we see in the stellar + HMXB simulation is, we examine several other simulations – two fully numerical simulations that include quasi-stellar object (QSO) sources, and two with contrasting source properties produced with the semi-numerical simulation 21CMFAST. We find the qualitative evolution of the normalized bispectrum during X-ray heating to be generic, unless the sources of X-rays are, as with QSOs, less numerous and so exhibit more distinct isolated heated profiles. Assuming mitigation of foreground and instrumental effects are ultimately effective, we find that we should be sensitive to the normalized bispectrum during the epoch of heating, so long as the spin temperature has not saturated by z≈19
Gridded and direct Epoch of Reionisation bispectrum estimates using the Murchison Widefield Array
We apply two methods to estimate the 21~cm bispectrum from data taken within
the Epoch of Reionisation (EoR) project of the Murchison Widefield Array (MWA).
Using data acquired with the Phase II compact array allows a direct bispectrum
estimate to be undertaken on the multiple redundantly-spaced triangles of
antenna tiles, as well as an estimate based on data gridded to the -plane.
The direct and gridded bispectrum estimators are applied to 21 hours of
high-band (167--197~MHz; =6.2--7.5) data from the 2016 and 2017 observing
seasons. Analytic predictions for the bispectrum bias and variance for point
source foregrounds are derived. We compare the output of these approaches, the
foreground contribution to the signal, and future prospects for measuring the
bispectra with redundant and non-redundant arrays. We find that some triangle
configurations yield bispectrum estimates that are consistent with the expected
noise level after 10 hours, while equilateral configurations are strongly
foreground-dominated. Careful choice of triangle configurations may be made to
reduce foreground bias that hinders power spectrum estimators, and the 21~cm
bispectrum may be accessible in less time than the 21~cm power spectrum for
some wave modes, with detections in hundreds of hours.Comment: 19 pages, 10 figures, accepted for publication in PAS