Robert J. Sault

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.

An H I Aperture Synthesis Mosaic of the Large Magellanic Cloud

We present the results of an H I aperture synthesis mosaic of the Large Magellanic Cloud (LMC), made by combining data from 1344 separate pointing centers using the Australia Telescope Compact Array (ATCA). The resolution of the mosaicked images is 10 (15 pc, using a distance to the LMC of 50 kpc). This mosaic, with a spatial resolution 15 times higher than that which had been previously obtained, emphasizes the turbulent and fractal structure of the ISM on the small scale, resulting from the dynamical feedback of the star formation processes with the ISM. The structure of the neutral atomic ISM in the LMC is dominated by H I filaments combined with numerous shells and holes. On the large scale, the H I disk appears to be remarkably symmetric and to have a well-organized and orderly, if somewhat complex, rotational field. The bulk of the H I resides in a disk 7.3 kpc in diameter. The mass of the disk component of the LMC is 2.5 × 109 M☉, and the upper limit to all mass within a radius of 4 kpc is ~3.5 × 109 M☉.

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.

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 Neutral Hydrogen Survey of the Large Magellanic Cloud: Aperture Synthesis and Multibeam Data Combined

Recent H I surveys of the Large Magellanic Cloud (LMC) with the Australia Telescope Compact Array and the Parkes multibeam receiver have focused, respectively, on the small-scale ( 1°) structure of the galaxy. Using a Fourier-plane technique, we have merged both data sets, providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc upward. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km s-1), brightness temperature sensitivity (2.4 K), and column density sensitivity (7.2 × 1018 cm-2 per 1.649 km s-1 channel) allow for studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbors to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. This paper presents the merged data and size spectrum of H I clouds, which is similar to the typical size spectrum of the holes and shells in the H I distribution. The H I clouds in the LMC have been identified by defining a cloud to be an object composed of all pixels in right ascension, declination, and velocity that are simply connected and that lie above the threshold brightness temperature.

Ultra-relativistic electrons in Jupiter's radiation belts

Ground-based observations have shown that Jupiter is a two-component source of microwave radio emission: thermal atmospheric emission and synchrotron emission from energetic electrons spiralling in Jupiters magnetic field. Later in situ measurements confirmed the existence of Jupiters high-energy electron-radiation belts, with evidence for electrons at energies up to 20 MeV. Although most radiation belt models predict electrons at higher energies, adiabatic diffusion theory can account only for energies up to around 20 MeV. Unambiguous evidence for more energetic electrons is lacking. Here we report observations of 13.8 GHz synchrotron emission that confirm the presence of electrons with energies up to 50 MeV; the data were collected during the Cassini fly-by of Jupiter. These energetic electrons may be repeatedly accelerated through an interaction with plasma waves, which can transfer energy into the electrons. Preliminary comparison of our data with model results suggests that electrons with energies of less than 20 MeV are more numerous than previously believed.

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz

We report here multiepoch, multifrequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic center. Data taken from the Very Large Array (VLA) archive indicate that the fractional circular polarization at 4.8 GHz was -0.31%, with an rms scatter of 0.13%, from 1981 to 1998, in spite of a factor of 2 change in the total intensity. The sign remained negative over the entire time range, indicating a stable magnetic field polarity. In the summer of 1999, we obtained 13 epochs of VLA A-configuration observations at 1.4, 4.8, 8.4, and 15 GHz. These observations employ a new technique that produces an error of 0.05% at 1.4, 4.8, and 8.4 GHz and 0.1% at 14.9 GHz. In 1999 May, September, and October, we obtained 11 epochs of Australia Telescope Compact Array (ATCA) observations at 4.8 and 8.5 GHz. In all three data sets, we find no evidence of linear polarization greater than 0.1%, in spite of strong circular polarization detections. Both VLA and ATCA data sets support three conclusions regarding the fractional circular polarization: (1) the average spectrum is inverted, with a spectral index α ≈ 0.5 ± 0.2; (2) the degree of variability is roughly constant on timescales of days to years; and (3) the degree of variability increases with frequency. We also observed that the largest increase in fractional circular polarization was coincident with the brightest flare in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hr was observed during this flare, indicating an upper limit to the intrinsic size during outburst of 70 AU at 15 GHz. The fractional circular polarization at 15 GHz reached -1.1%, and the spectral index was strongly inverted (α ~ 1.5) during this flare. We tentatively conclude that the spectrum has two components that match the high- and low-frequency total intensity components.

Real-Time Calibration of the Murchison Widefield Array

The interferometric technique known as peeling addresses many of the challenges faced when observing with low-frequency radio arrays, and is a promising tool for the associated calibration systems. We investigate a real-time peeling implementation for next-generation radio interferometers such as the Murchison widefield array (MWA). The MWA is being built in Australia and will observe the radio sky between 80 and 300 MHz. The data rate produced by the correlator is just over 19 GB/s (a few peta-bytes/day). It is impractical to store data generated at this rate, and software is currently being developed to calibrate and form images in real time. The software will run on-site on a high-throughput real-time computing cluster at several tera-flops, and a complete cycle of calibration and imaging will be completed every 8 s. Various properties of the implementation are investigated using simulated data. The algorithm is seen to work in the presence of strong galactic emission and with various ionospheric conditions. It is also shown to scale well as the number of antennas increases, which is essential for many upcoming instruments. Lessons from MWA pipeline development and processing of simulated data may be applied to future low-frequency fixed dipole arrays.

Light Curves and Radio Structure of the 1999 September Transient Event in V4641 Sagittarii (=XTE J1819-254=SAX J1819.3-2525)

We report on radio observations of the 1999 September event of the X-ray transient V4641 Sgr (=XTE J1819-254=SAX J1819.3-2525). This event was extremely rapid in its rise and decay across radio, optical, and X-ray wavelengths; the X-rays rose to 12 crab within 8 hr and faded to below 0.1 crab in less than 2 hr. Radio observations were made with seven telescopes during the first day following the onset of the strong X-ray event, revealing a strong radio source that was detected for 3 further weeks by the more sensitive telescopes. The radio source was resolved even in the first Very Large Array (VLA) images (September 16.027 UT), being ~025 long with an axis ratio of at least 10 : 1. The total flux density decayed by a factor of ~4 over the first day, and by September 17.94 UT the radio emission was confined to a slowly decaying, marginally resolved remnant located at one side of the early elongated emission. The H I absorption spectrum gives a minimum kinematic distance of about 400 pc; various other arguments suggest that the true distance is not much greater than this. The inferred proper motions for the early extended emission (04-11 day-1) correspond to v/c ~ 1.0-3.2 (d/0.5 kpc), and this together with the radio morphology argues that this is a relativistic jet source like GRS 1915+105 and GRO J1655-40. The proper motion of the late-time remnant is at least 100 times smaller. One simple interpretation posits the ejection of a single short-lived jet segment, followed by a more slowly decaying, optically thin jet segment ejection. These two components can explain both the multifrequency radio light curves and the radio images. The most likely parameters for the fast-jet system with net-averaged proper motion of ~04 day-1, assuming d = 0.5 kpc, are v ~ 0.85c and i ~ 63°, where i is the inclination to the line of sight. The corresponding apparent velocities are 1.4c and 0.6c for the approaching and receding jets, making V4641 Sgr the closest superluminal jet source known.