42 research outputs found
SARAS: a precision system for measurement of the Cosmic Radio Background and signatures from the Epoch of Reionization
SARAS is a correlation spectrometer purpose designed for precision
measurements of the cosmic radio background and faint features in the sky
spectrum at long wavelengths that arise from redshifted 21-cm from gas in the
reionization epoch. SARAS operates in the octave band 87.5-175 MHz. We present
herein the system design arguing for a complex correlation spectrometer
concept. The SARAS design concept provides a differential measurement between
the antenna temperature and that of an internal reference termination, with
measurements in switched system states allowing for cancellation of additive
contaminants from a large part of the signal flow path including the digital
spectrometer. A switched noise injection scheme provides absolute spectral
calibration. Additionally, we argue for an electrically small
frequency-independent antenna over an absorber ground. Various critical design
features that aid in avoidance of systematics and in providing calibration
products for the parametrization of other unavoidable systematics are described
and the rationale discussed. The signal flow and processing is analyzed and the
response to noise temperatures of the antenna, reference termination and
amplifiers is computed. Multi-path propagation arising from internal
reflections are considered in the analysis, which includes a harmonic series of
internal reflections. We opine that the SARAS design concept is advantageous
for precision measurement of the absolute cosmic radio background spectrum;
therefore, the design features and analysis methods presented here are expected
to serve as a basis for implementations tailored to measurements of a
multiplicity of features in the background sky at long wavelengths, which may
arise from events in the dark ages and subsequent reionization era.Comment: 49 pages, 17 figure
Effects of Antenna Beam Chromaticity on Redshifted 21~cm Power Spectrum and Implications for Hydrogen Epoch of Reionization Array
Unaccounted for systematics from foregrounds and instruments can severely
limit the sensitivity of current experiments from detecting redshifted 21~cm
signals from the Epoch of Reionization (EoR). Upcoming experiments are faced
with a challenge to deliver more collecting area per antenna element without
degrading the data with systematics. This paper and its companions show that
dishes are viable for achieving this balance using the Hydrogen Epoch of
Reionization Array (HERA) as an example. Here, we specifically identify
spectral systematics associated with the antenna power pattern as a significant
detriment to all EoR experiments which causes the already bright foreground
power to leak well beyond ideal limits and contaminate the otherwise clean EoR
signal modes. A primary source of this chromaticity is reflections in the
antenna-feed assembly and between structures in neighboring antennas. Using
precise foreground simulations taking wide-field effects into account, we
provide a framework to set cosmologically-motivated design specifications on
these reflections to prevent further EoR signal degradation. We show HERA will
not be impeded by such spectral systematics and demonstrate that even in a
conservative scenario that does not perform removal of foregrounds, HERA will
detect EoR signal in line-of-sight -modes, ~Mpc, with high significance. All baselines in a 19-element HERA
layout are capable of detecting EoR over a substantial observing window on the
sky.Comment: 11 pages, 6 figures (10 total including subfigures), submitted to Ap
The Hydrogen Epoch of Reionization Array Dish II: Characterization of Spectral Structure with Electromagnetic Simulations and its science Implications
We use time-domain electromagnetic simulations to determine the spectral
characteristics of the Hydrogen Epoch of Reionization Arrays (HERA) antenna.
These simulations are part of a multi-faceted campaign to determine the
effectiveness of the dish's design for obtaining a detection of redshifted 21
cm emission from the epoch of reionization. Our simulations show the existence
of reflections between HERA's suspended feed and its parabolic dish reflector
that fall below -40 dB at 150 ns and, for reasonable impedance matches, have a
negligible impact on HERA's ability to constrain EoR parameters. It follows
that despite the reflections they introduce, dishes are effective for
increasing the sensitivity of EoR experiments at relatively low cost. We find
that electromagnetic resonances in the HERA feed's cylindrical skirt, which is
intended to reduce cross coupling and beam ellipticity, introduces significant
power at large delays ( dB at 200 ns) which can lead to some loss of
measurable Fourier modes and a modest reduction in sensitivity. Even in the
presence of this structure, we find that the spectral response of the antenna
is sufficiently smooth for delay filtering to contain foreground emission at
line-of-sight wave numbers below Mpc, in
the region where the current PAPER experiment operates. Incorporating these
results into a Fisher Matrix analysis, we find that the spectral structure
observed in our simulations has only a small effect on the tight constraints
HERA can achieve on parameters associated with the astrophysics of
reionization.Comment: Accepted to ApJ, 18 pages, 17 Figures. Replacement matches accepted
manuscrip
The Hydrogen Epoch of Reionization Array Dish I: Beam Pattern Measurements and Science Implications
The Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer
aiming to detect the power spectrum of 21 cm fluctuations from neutral hydrogen
from the Epoch of Reionization (EOR). Drawing on lessons from the Murchison
Widefield Array (MWA) and the Precision Array for Probing the Epoch of
Reionization (PAPER), HERA is a hexagonal array of large (14 m diameter) dishes
with suspended dipole feeds. Not only does the dish determine overall
sensitivity, it affects the observed frequency structure of foregrounds in the
interferometer. This is the first of a series of four papers characterizing the
frequency and angular response of the dish with simulations and measurements.
We focus in this paper on the angular response (i.e., power pattern), which
sets the relative weighting between sky regions of high and low delay, and
thus, apparent source frequency structure. We measure the angular response at
137 MHz using the ORBCOMM beam mapping system of Neben et al. We measure a
collecting area of 93 m^2 in the optimal dish/feed configuration, implying
HERA-320 should detect the EOR power spectrum at z~9 with a signal-to-noise
ratio of 12.7 using a foreground avoidance approach with a single season of
observations, and 74.3 using a foreground subtraction approach. Lastly we study
the impact of these beam measurements on the distribution of foregrounds in
Fourier space.Comment: 13 pages, 9 figures. Replaced to match accepted ApJ versio
Hydrogen Epoch of Reionization Array (HERA)
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to
measure 21 cm emission from the primordial intergalactic medium (IGM)
throughout cosmic reionization (), and to explore earlier epochs of our
Cosmic Dawn (). During these epochs, early stars and black holes
heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is
designed to characterize the evolution of the 21 cm power spectrum to constrain
the timing and morphology of reionization, the properties of the first
galaxies, the evolution of large-scale structure, and the early sources of
heating. The full HERA instrument will be a 350-element interferometer in South
Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz.
Currently, 19 dishes have been deployed on site and the next 18 are under
construction. HERA has been designated as an SKA Precursor instrument.
In this paper, we summarize HERA's scientific context and provide forecasts
for its key science results. After reviewing the current state of the art in
foreground mitigation, we use the delay-spectrum technique to motivate
high-level performance requirements for the HERA instrument. Next, we present
the HERA instrument design, along with the subsystem specifications that ensure
that HERA meets its performance requirements. Finally, we summarize the
schedule and status of the project. We conclude by suggesting that, given the
realities of foreground contamination, current-generation 21 cm instruments are
approaching their sensitivity limits. HERA is designed to bring both the
sensitivity and the precision to deliver its primary science on the basis of
proven foreground filtering techniques, while developing new subtraction
techniques to unlock new capabilities. The result will be a major step toward
realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table
HI 21cm Cosmology and the Bi-spectrum: Closure Diagnostics in Massively Redundant Interferometric Arrays
New massively redundant low frequency arrays allow for a novel investigation
of closure relations in interferometry. We employ commissioning data from the
Hydrogen Epoch of Reionization Array to investigate closure quantities in this
densely packed grid array of 14m antennas operating at 100 MHz to 200 MHz. We
investigate techniques that utilize closure phase spectra for redundant triads
to estimate departures from redundancy for redundant baseline visibilities. We
find a median absolute deviation from redundancy in closure phase across the
observed frequency range of about 4.5deg. This value translates into a
non-redundancy per visibility phase of about 2.6deg, using prototype
electronics. The median absolute deviations from redundancy decrease with
longer baselines. We show that closure phase spectra can be used to identify
ill-behaved antennas in the array, independent of calibration. We investigate
the temporal behavior of closure spectra. The Allan variance increases after a
one minute stride time, due to passage of the sky through the primary beam of
the transit telescope. However, the closure spectra repeat to well within the
noise per measurement at corresponding local sidereal times (LST) from day to
day. In future papers in this series we will develop the technique of using
closure phase spectra in the search for the HI 21cm signal from cosmic
reionization.Comment: 32 pages. 11 figures. Accepted to Radio Scienc
Optimizing Sparse RFI Prediction using Deep Learning
Radio Frequency Interference (RFI) is an ever-present limiting factor among
radio telescopes even in the most remote observing locations. When looking to
retain the maximum amount of sensitivity and reduce contamination for Epoch of
Reionization studies, the identification and removal of RFI is especially
important. In addition to improved RFI identification, we must also take into
account computational efficiency of the RFI-Identification algorithm as radio
interferometer arrays such as the Hydrogen Epoch of Reionization Array grow
larger in number of receivers. To address this, we present a Deep Fully
Convolutional Neural Network (DFCN) that is comprehensive in its use of
interferometric data, where both amplitude and phase information are used
jointly for identifying RFI. We train the network using simulated HERA
visibilities containing mock RFI, yielding a known "ground truth" dataset for
evaluating the accuracy of various RFI algorithms. Evaluation of the DFCN model
is performed on observations from the 67 dish build-out, HERA-67, and achieves
a data throughput of 1.6 HERA time-ordered 1024 channeled
visibilities per hour per GPU. We determine that relative to an amplitude only
network including visibility phase adds important adjacent time-frequency
context which increases discrimination between RFI and Non-RFI. The inclusion
of phase when predicting achieves a Recall of 0.81, Precision of 0.58, and
score of 0.75 as applied to our HERA-67 observations.Comment: 11 pages, 7 figure
Mitigating Internal Instrument Coupling for 21 cm Cosmology. II. A Method Demonstration with the Hydrogen Epoch of Reionization Array
We present a study of internal reflection and cross-coupling systematics in Phase I of the Hydrogen Epoch of Reionization Array (HERA). In a companion paper, we outlined the mathematical formalism for such systematics and presented algorithms for modeling and removing them from the data. In this work, we apply these techniques to data from HERA's first observing season as a method demonstration. The data show evidence for systematics that, without removal, would hinder a detection of the 21 cm power spectrum for the targeted Epoch of Reionization (EoR) line-of-sight modes in the range 0.2 h −1 Mpc−1 < < 0.5 h −1 Mpc−1. In particular, we find evidence for nonnegligible amounts of spectral structure in the raw autocorrelations that overlaps with the EoR window and is suggestive of complex instrumental effects. Through systematic modeling on a single night of data, we find we can recover these modes in the power spectrum down to the integrated noise floor, achieving a dynamic range in the EoR window of 106 in power (mK2 units) with respect to the bright galactic foreground signal. Future work with deeper integrations will help determine whether these systematics can continue to be mitigated down to EoR levels. For future observing seasons, HERA will have upgraded analog and digital hardware to better control these systematics in the field
Detection of Cosmic Structures using the Bispectrum Phase. II. First Results from Application to Cosmic Reionization Using the Hydrogen Epoch of Reionization Array
Characterizing the epoch of reionization (EoR) at via the
redshifted 21 cm line of neutral Hydrogen (HI) is critical to modern
astrophysics and cosmology, and thus a key science goal of many current and
planned low-frequency radio telescopes. The primary challenge to detecting this
signal is the overwhelmingly bright foreground emission at these frequencies,
placing stringent requirements on the knowledge of the instruments and
inaccuracies in analyses. Results from these experiments have largely been
limited not by thermal sensitivity but by systematics, particularly caused by
the inability to calibrate the instrument to high accuracy. The interferometric
bispectrum phase is immune to antenna-based calibration and errors therein, and
presents an independent alternative to detect the EoR HI fluctuations while
largely avoiding calibration systematics. Here, we provide a demonstration of
this technique on a subset of data from the Hydrogen Epoch of Reionization
Array (HERA) to place approximate constraints on the brightness temperature of
the intergalactic medium (IGM). From this limited data, at we infer
"" upper limits on the IGM brightness temperature to be
"pseudo" mK at "pseudo" Mpc (data-limited)
and "pseudo" mK at "pseudo" Mpc
(noise-limited). The "pseudo" units denote only an approximate and not an exact
correspondence to the actual distance scales and brightness temperatures. By
propagating models in parallel to the data analysis, we confirm that the
dynamic range required to separate the cosmic HI signal from the foregrounds is
similar to that in standard approaches, and the power spectrum of the
bispectrum phase is still data-limited (at dynamic range)
indicating scope for further improvement in sensitivity as the array build-out
continues.Comment: 22 pages, 12 figures (including sub-figures). Published in PhRvD.
Abstract may be slightly abridged compared to the actual manuscript due to
length limitations on arXi