85 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 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
Polarized Redundant-Baseline Calibration for 21 cm Cosmology Without Adding Spectral Structure
21 cm cosmology is a promising new probe of the evolution of visible matter
in our universe, especially during the poorly-constrained Cosmic Dawn and Epoch
of Reionization. However, in order to separate the 21 cm signal from bright
astrophysical foregrounds, we need an exquisite understanding of our telescopes
so as to avoid adding spectral structure to spectrally-smooth foregrounds. One
powerful calibration method relies on repeated simultaneous measurements of the
same interferometric baseline to solve for the sky signal and for instrumental
parameters simultaneously. However, certain degrees of freedom are not
constrained by asserting internal consistency between redundant measurements.
In this paper, we review the origin of these "degeneracies" of
redundant-baseline calibration and demonstrate how they can source unwanted
spectral structure in our measurement and show how to eliminate that
additional, artificial structure. We also generalize redundant calibration to
dual-polarization instruments, derive the degeneracy structure, and explore the
unique challenges to calibration and preserving spectral smoothness presented
by a polarized measurement.Comment: 12 pages, 3 figures, updated to match the published MNRAS versio
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