147 research outputs found
Upper Limits on the 21 cm Power Spectrum at z = 5.9 from Quasar Absorption Line Spectroscopy
We present upper limits on the 21 cm power spectrum at calculated
from the model-independent limit on the neutral fraction of the intergalactic
medium of derived from dark
pixel statistics of quasar absorption spectra. Using 21CMMC, a Markov chain
Monte Carlo Epoch of Reionization analysis code, we explore the probability
distribution of 21 cm power spectra consistent with this constraint on the
neutral fraction. We present 99 per cent confidence upper limits of
to over a range of from 0.5 to $2.0\
h{\rm Mpc}^{-1}kz=5.9$ in excess of this value is highly suggestive of residual foreground
contamination or other systematic errors affecting the analysis.Comment: 5 pages, 1 figure, accepted to MNRAS letter
Constraints on the temperature of the intergalactic medium at z=8.4 with 21-cm observations
We compute robust lower limits on the spin temperature, , of the
intergalactic medium (IGM), implied by the upper limits on the 21-cm
power spectrum recently measured by PAPER-64. Unlike previous studies which
used a single epoch of reionization (EoR) model, our approach samples a large
parameter space of EoR models: the dominant uncertainty when estimating
constraints on . Allowing to be a free parameter and
marginalizing over EoR parameters in our Markov Chain Monte Carlo code 21CMMC,
we infer (corresponding approximately to ) for
a mean IGM neutral fraction of . We
further improve on these limits by folding-in additional EoR constraints based
on: (i) the dark fraction in QSO spectra, which implies a strict upper limit of
; and (ii) the
electron scattering optical depth,
measured by the Planck satellite. By restricting the allowed EoR models, these
additional observations tighten the approximate lower limits on the
spin temperature to K. Thus, even such preliminary 21-cm
observations begin to rule out extreme scenarios such as `cold reionization',
implying at least some prior heating of the IGM. The analysis framework
developed here can be applied to upcoming 21-cm observations, thereby providing
unique insights into the sources which heated and subsequently reionized the
very early Universe.Comment: 7 pages, 1 figure, accepted to MNRAS (matches online version
A Sensitivity and Array-Configuration Study for Measuring the Power Spectrum of 21cm Emission from Reionization
Telescopes aiming to measure 21cm emission from the Epoch of Reionization
must toe a careful line, balancing the need for raw sensitivity against the
stringent calibration requirements for removing bright foregrounds. It is
unclear what the optimal design is for achieving both of these goals. Via a
pedagogical derivation of an interferometer's response to the power spectrum of
21cm reionization fluctuations, we show that even under optimistic scenarios,
first-generation arrays will yield low-SNR detections, and that different
compact array configurations can substantially alter sensitivity. We explore
the sensitivity gains of array configurations that yield high redundancy in the
uv-plane -- configurations that have been largely ignored since the advent of
self-calibration for high-dynamic-range imaging. We first introduce a
mathematical framework to generate optimal minimum-redundancy configurations
for imaging. We contrast the sensitivity of such configurations with
high-redundancy configurations, finding that high-redundancy configurations can
improve power-spectrum sensitivity by more than an order of magnitude. We
explore how high-redundancy array configurations can be tuned to various
angular scales, enabling array sensitivity to be directed away from regions of
the uv-plane (such as the origin) where foregrounds are brighter and where
instrumental systematics are more problematic. We demonstrate that a
132-antenna deployment of the Precision Array for Probing the Epoch of
Reionization (PAPER) observing for 120 days in a high-redundancy configuration
will, under ideal conditions, have the requisite sensitivity to detect the
power spectrum of the 21cm signal from reionization at a 3\sigma level at
k<0.25h Mpc^{-1} in a bin of \Delta ln k=1. We discuss the tradeoffs of low-
versus high-redundancy configurations.Comment: 34 pages, 5 figures, 2 appendices. Version accepted to Ap
A Bayesian approach to high fidelity interferometric calibration II: demonstration with simulated data
In a companion paper, we presented BayesCal, a mathematical formalism for
mitigating sky-model incompleteness in interferometric calibration. In this
paper, we demonstrate the use of BayesCal to calibrate the degenerate gain
parameters of full-Stokes simulated observations with a HERA-like hexagonal
close-packed redundant array, for three assumed levels of completeness of the a
priori known component of the calibration sky model. We compare the BayesCal
calibration solutions to those recovered by calibrating the degenerate gain
parameters with only the a priori known component of the calibration sky model
both with and without imposing physically motivated priors on the gain
amplitude solutions and for two choices of baseline length range over which to
calibrate. We find that BayesCal provides calibration solutions with up to four
orders of magnitude lower power in spurious gain amplitude fluctuations than
the calibration solutions derived for the same data set with the alternate
approaches, and between and times smaller than in the
mean degenerate gain amplitude on the full range of spectral scales accessible
in the data. Additionally, we find that in the scenarios modelled only BayesCal
has sufficiently high fidelity calibration solutions for unbiased recovery of
the 21 cm power spectrum on large spectral scales (). In all other cases, in the completeness regimes
studied, those scales are contaminated
Constraining High Redshift X-ray Sources with Next Generation 21 cm Power Spectrum Measurements
We use the Fisher matrix formalism and semi-numerical simulations to derive
quantitative predictions of the constraints that power spectrum measurements on
next-generation interferometers, such as the Hydrogen Epoch of Reionization
Array (HERA) and the Square Kilometre Array (SKA), will place on the
characteristics of the X-ray sources that heated the high redshift
intergalactic medium. Incorporating observations between and , we
find that the proposed 331 element HERA and SKA phase 1 will be capable of
placing constraints on the spectral properties of these first
X-ray sources, even if one is unable to perform measurements within the
foreground contaminated "wedge" or the FM band. When accounting for the
enhancement in power spectrum amplitude from spin temperature fluctuations, we
find that the observable signatures of reionization extend well beyond the peak
in the power spectrum usually associated with it. We also find that lower
redshift degeneracies between the signatures of heating and reionization
physics lead to errors on reionization parameters that are significantly
greater than previously predicted. Observations over the heating epoch are able
to break these degeneracies and improve our constraints considerably. For these
two reasons, 21\,cm observations during the heating epoch significantly enhance
our understanding of reionization as well.Comment: 15 pages, 10 figures, Accepted to MNRA
The Statistics of Negative Power Spectrum Systematics in some 21 cm Analyses
Through a very careful analysis Kolopanis et al. (2022) identified a negative
power spectrum (PS) systematic. The 21 cm cosmology community has assumed that
any observational systematics would add power, as negative PS are non-physical.
In addition to the mystery of their origin, negative PS systematics raise the
spectre of artificially lowering upper limits on the 21 cm PS. It appears that
the source of the negative PS systematics is a subtle interaction between
choices in how the PS estimate is calculated and baseline-dependent systematic
power. In this paper we present a statistical model of baseline dependent
systematics to explore how negative PS systematics can appear and their
statistical characteristics. This leads us to recommendations on when and how
to consider negative PS systematics when reporting observational 21 cm
cosmology upper limit.Comment: Submitted to MNRA
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