Judd D. Bowman

The Murchison Widefield Array: The Square Kilometre Array Precursor at Low Radio Frequencies

The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80–300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ~3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

A lower limit of δz>0.06 for the duration of the reionization epoch

Observations of the 21-centimetre line of atomic hydrogen in the early Universe directly probe the history of the reionization of the gas between galaxies. The observations are challenging, though, because of the low expected signal strength (∼10 mK), and contamination by strong (>100 K) foreground synchrotron emission in the Milky Way and extragalactic continuum sources. If reionization happened rapidly, there should be a characteristic signature visible against the smooth foreground in an all-sky spectrum. Here we report an all-sky spectrum between 100 and 200 MHz, corresponding to the redshift range 6 < z < 13 for the 21-centimetre line. The data exclude a rapid reionization timescale of Δz < 0.06 at the 95% confidence level.

THE SENSITIVITY OF FIRST-GENERATION EPOCH OF REIONIZATION OBSERVATORIES AND THEIR POTENTIAL FOR DIFFERENTIATING THEORETICAL POWER SPECTRA

Statistical observations of the epoch of reionization (EOR) power spectrum provide a rich data set for understanding the transition from the cosmic ‘‘dark ages’’ to the ionized universe we see today. EOR observations have become an active area of experimental cosmology, and three first-generation observatories—MWA, PAST, and LOFAR—are currently under development. In this paper we provide the first quantitative calculation of the threedimensional power spectrum sensitivity, incorporating thedesign parameters of a planned array. This calculation is then used to explore the constraints these first-generation observations can place on the EOR power spectrum. The results demonstrate the potential for upcoming power spectrum observations to constrain theories of structure formation and reionization. Subject headings: early universe — intergalactic medium — radio lines: general — techniques: interferometric Online material: color figure

Bright Source Subtraction Requirements for Redshifted 21 cm Measurements

The H I 21 cm transition line is expected to be an important probe into the cosmic dark ages and epoch of reionization. Foreground source removal is one of the principal challenges for the detection of this signal. This paper investigates the extragalactic point source contamination and how accurately bright sources (≳ Jy) must be removed in order to detect 21 cm emission with upcoming radio telescopes such as the Murchison Widefield Array. We consider the residual contamination in 21 cm maps and power spectra due to position errors in the sky model for bright sources, as well as frequency-independent calibration errors. We find that a source position accuracy of 0.1 arcsec will suffice for detection of the H I power spectrum. For calibration errors, 0.05% accuracy in antenna gain amplitude is required in order to detect the cosmic signal. Both sources of subtraction error produce residuals that are localized to small angular scales, k⊥ ≳ 0.05 Mpc^(–1), in the two-dimensional power spectrum.The HI 21 cm transition line is expected to be an important probe int o the cosmic dark ages and epoch of reionization. Foreground source removal is one of the princ ipal challenges for the detection of this signal. This paper investigates the extragalactic point source contami tion and how accurately bright sources ( & 1 Jy) must be removed in order to detect 21 cm emission with upcomin g radio telescopes such as the Murchison Widefield Array (MWA). We consider the residual contaminati on in 21 cm maps and power spectra due to position errors in the sky-model for bright sources, as well as frequency independent calibration errors. We find that a source position accuracy of 0 .1 arcsec will suffice for detection of the H I power spectrum. For calibration errors, 0 .05 % accuracy in antenna gain amplitude is required in order t o de ect the cosmic signal. Both sources of subtraction error produce residuals that ar e localized to small angular scales, k⊥ & 0.05 Mpc−1, in the two-dimensional power spectrum. Subject headings: Cosmology: Early Universe, Galaxies: Intergalactic Mediu m, Radio Lines: General, Techniques: Interferometric, Methods: Data Analysis

FOREGROUND CONTAMINATION IN INTERFEROMETRIC MEASUREMENTS OF THE REDSHIFTED 21 cm POWER SPECTRUM

Subtraction of astrophysical foreground contamination from “dirty” sky maps produced by simulated measurements of the Murchison Widefield Array (MWA) has been performed by fitting a third-order polynomial along the spectral dimension of each pixel in the data cubes. The simulations are the first to include the unavoidable instrumental effects of the frequency-dependent primary antenna beams and synthesized array beams. They recover the onedimensional spherically binned input redshifted 21 cm power spectrum within ∼ 1% over the scales probed most sensitively by the MWA (0.01 k 1M pc −1 ) and demonstrate that realistic instrumental effects will not mask the epoch of reionization signal. We find that the weighting function used to produce the dirty sky maps from the gridded visibility measurements is important to the success of the technique. Uniform weighting of the visibility measurements produces the best results, whereas natural weighting significantly worsens the foreground subtraction by coupling structure in the density of the visibility measurements to spectral structure in the dirty sky map data cube. The extremely dense uv-coverage of the MWA was found to be advantageous for this technique and produced very good results on scales corresponding to |u| 500λ in the uv-plane without any selective editing of the uv-coverage.

wsclean: an implementation of a fast, generic wide-field imager for radio astronomy

Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASAs w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.

Probing the first stars and black holes in the early Universe with the Dark Ages Radio Explorer (DARE)

Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a per vasive, diffuse component of the intergalactic medium (IGM). This transition tracks the glo bal neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) br ightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but a t e rlier times (10< z < 25), the mean brightness temperature should probe the spin temperat ur of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature o f the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the e arly evolution of galaxies through the heating and ionizing of the IGM by their stellar populations . Experiments are already underway to accomplish this task or, at least, provide basic constraint s o the evolution of the mean brightness temperature. We provide a brief overview of one of these proj ects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for fut u e results.

Improving Foreground Subtraction in Statistical Observations of 21 cm Emission from the Epoch of Reionization

Statistical observations of the epoch of reionization using the 21 cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground-removal techniques have been proposed in the literature, and it has been assumed that these would be used in combination to reveal the epoch of reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground-removal techniques, we identify an additional subtraction stage that can further reduce the EOR foreground contamination, and study the interactions between the foreground-removal algorithms. This enables us to outline a comprehensive foreground-removal strategy that incorporates all previously proposed subtraction techniques. Using this foreground-removal framework and the characteristic subtraction errors, we discuss the complementarity of different foreground-removal techniques and the implications for array design and the analysis of EOR data.

An improved method for 21-cm foreground removal

21-cm tomography is expected to be difficult in part because of serious foreground contamination. Previous studies have found that line-of-sight approaches are capable of cleaning foregrounds to an acceptable level on large spatial scales, but not on small spatial scales. In this paper, we introduce a Fourier space formalism for describing the line-of-sight methods, and use it to introduce an improved new method for 21-cm foreground cleaning. Heuristically, this method involves fitting foregrounds in Fourier space using weighted polynomial fits, with each pixel weighted according to its information content. We show that the new method reproduces the old one on large angular scales, and gives marked improvements on small scales at essentially no extra computational cost.