David Herne

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.

The Murchison Widefield Array: Design Overview

The Murchison Widefield Array is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations but is initially focused on three key science projects: detection and characterization of three-dimensional brightness temperature fluctuations in the 21 cm line of neutral hydrogen during the epoch of reionization (EoR) at redshifts from six to ten; solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources; and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broadband active dipoles, arranged into 512 ldquotilesrdquo comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5 km in diameter, with a small number of outliers extending to 3 km. All tile-tile baselines are correlated in custom field-programmable gate array based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment, allowing full exploitation of the instrumental capabilities.

The Precision Array for Probing the Epoch of Re-ionization: Eight Station Results

We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e−5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (C� ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at � = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.

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 study of fundamental limitations to statistical detection of redshifted H i from the epoch of reionization

In this paper, we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise, and sample variance, in detecting the H I power spectrum from the epoch of reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the H I power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k -space. Sensitivity for EoR H I power spectrum detection is estimated for different observing modes with MWA. With 1000 hr of observing on a single field using the 128 tile MWA, EoR detection is feasible (S/N >1 for k ≲ 0.8 Mpc -1 ). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.

The EoR sensitivity of the murchison widefield array

Using the final 128 antenna locations of the MurchisonWidefield Array (MWA), we calculate its sensitivity to the epoch of reionization (EoR) power spectrum of redshifted 21 cm emission for a fiducial model and provide the tools to calculate the sensitivity for any model. Our calculation takes into account synthesis rotation, chromatic and asymmetrical baseline effects, and excludes modes that will be contaminated by foreground subtraction. For the fiducial model, the MWA will be capable of a 14 σ detection of the EoR signal with one full season of observation on two fields (900 and 700 h).

Field Deployment of Prototype Antenna Tiles for the Mileura Widefield Array Low Frequency Demonstrator

Experiments were performed with prototype antenna tiles for the Mileura Widefield Array Low Frequency Demonstrator (MWA LFD) to better understand the wide-field, wide-band properties of their design and to characterize the radio-frequency interference (RFI) between 80 and 300 MHz at the site in Western Australia. Observations acquired during the 6 month deployment confirmed the predicted sensitivity of the antennas, sky-noise-dominated systemtemperatures,andphase-coherentinterferometricmeasurements.Theradiospectrumisremarkablyfreeofstrong terrestrial signals, with the exception of two narrow frequency bands allocated to satellite downlinks, and rare bursts duetoground-based transmissionsbeingscatteredfrom aircraft andmeteortrails. Resultsindicatethepotential ofthe MWA LFD to make significant achievements in its three key science objectives: epoch of reionization science, heliospheric science, and radio transient detection.

Fast Holographic Deconvolution: a new technique for precision radio interferometry

We introduce the Fast Holographic Deconvolution method for analyzing interferometric radio data. Our new method is an extension of A-projection/software-holography/forward modeling analysis techniques and shares their precision deconvolution and wide-field polarimetry, while being significantly faster than current implementations that use full direction-dependent antenna gains. Using data from the MWA 32 antenna prototype, we demonstrate the effectiveness and precision of our new algorithm. Fast Holographic Deconvolution may be particularly important for upcoming 21 cm cosmology observations of the Epoch of Reionization and Dark Energy where foreground subtraction is intimately related to the precision of the data reduction.

Interferometric Imaging with the 32 Element Murchison Wide-Field Array

The Murchison Wide-Field Array (MWA) is a low-frequency radio telescope, currently under construction, intended to search for the spectral signature of the epoch of reionization (EOR) and to probe the structure of the solar corona. Sited in western Australia, the full MWA will comprise 8192 dipoles grouped into 512 tiles and will be capable of imaging the sky south of 40° declination, from 80 MHz to 300 MHz with an instantaneous field of view that is tens of degrees wide and a resolution of a few arcminutes. A 32 station prototype of the MWA has been recently commissioned and a set of observations has been taken that exercise the whole acquisition and processing pipeline. We present Stokes I, Q, and U images from two ~4 hr integrations of a field 20° wide centered on Pictoris A. These images demonstrate the capacity and stability of a real-time calibration and imaging technique employing the weighted addition of warped snapshots to counter extreme wide-field imaging distortions.

A survey for transients and variables with the Murchison Widefield Array 32-tile prototype at 154 MHz

We present a search for transient and variable radio sources at 154 MHz with the Murchison Widefield Array 32-tile prototype. 51 images were obtained that cover a field of view of 1430 deg 2 centred on Hydra A. The observations were obtained over three days in 2010 March and three days in 2011 April and May. The mean cadence of the observations was 26 min and there was additional temporal information on day and year time-scales.