arXiv subjects

R. J. Sault

Publications and source records attributed to R. J. Sault.

At least 37 records · Page 2Linked to original sources

arXiv2013

The Murchison Widefield Array: solar science with the low frequency SKA Precursor

The Murchison Widefield Array is a low frequency (80 - 300 MHz) SKA Precursor, comprising 128 aperture array elements (known as tiles) distributed over an area of 3 km diameter. The MWA is located at the extraordinarily radio quiet Murchison Radioastronomy Observatory in the mid-west of Western Australia, the selected home for the Phase 1 and Phase 2 SKA low frequency arrays. The MWA science goals include: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionisation; 2) studies of Galactic and extragalactic processes based on deep, confusion-limited surveys of the full sky visible to the array; 3) time domain astrophysics through exploration of the variable radio sky; and 4) solar imaging and characterisation of the heliosphere and ionosphere via propagation effects on background radio source emission. This paper concentrates on the capabilities of the MWA for solar science and summarises some of the solar science results to date, in advance of the initial operation of the final instrument in 2013.

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arXiv2012

Realisation of a low frequency SKA Precursor: The Murchison Widefield Array

The Murchison Widefield Array is a low frequency (80 - 300 MHz) SKA Precursor, comprising 128 aperture array elements distributed over an area of 3 km diameter. The MWA is located at the extraordinarily radio quiet Murchison Radioastronomy Observatory in the mid-west of Western Australia, the selected home for the Phase 1 and Phase 2 SKA low frequency arrays. The MWA science goals include: 1) detection of fluctuations in the brightness temperature of the diffuse redshifted 21 cm line of neutral hydrogen from the epoch of reionisation; 2) studies of Galactic and extragalactic processes based on deep, confusion-limited surveys of the full sky visible to the array; 3) time domain astrophysics through exploration of the variable radio sky; and 4) solar imaging and characterisation of the heliosphere and ionosphere via propagation effects on background radio source emission. This paper will focus on a brief discussion of the as-built MWA system, highlighting several novel characteristics of the instrument, and a brief progress report (as of June 2012) on the final construction phase. Practical completion of the MWA is expected in November 2012, with commissioning commencing from approximately August 2012 and operations commencing near mid 2013. A brief description of recent science results from the MWA prototype instrument is given.

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arXiv2012

Low Frequency Observations of the Moon with the Murchison Widefield Array

A new generation of low frequency radio telescopes is seeking to observe the redshifted 21 cm signal from the Epoch of Reionization (EoR), requiring innovative methods of calibration and imaging to overcome the difficulties of widefield low frequency radio interferometry. Precise calibration will be required to separate the small expected EoR signal from the strong foreground emission at the frequencies of interest between 80 and 300 MHz. The Moon may be useful as a calibration source for detection of the EoR signature, as it should have a smooth and predictable thermal spectrum across the frequency band of interest. Initial observations of the Moon with the Murchison Widefield Array 32 tile prototype show that the Moon does exhibit a similar trend to that expected for a cool thermally emitting body in the observed frequency range, but that the spectrum is corrupted by reflected radio emission from Earth. In particular, there is an abrupt increase in the observed flux density of the Moon within the internationally recognised Frequency Modulated (FM) radio band. The observations have implications for future low frequency surveys and EoR detection experiments that will need to take this reflected emission from the Moon into account. The results also allow us to estimate the equivalent isotropic power emitted by the Earth in the FM band and to determine how bright the Earth might appear at metre wavelengths to an observer beyond our own solar system.

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arXiv2012

The EoR Sensitivity of the Murchison Widefield Array

Using the final 128 antenna locations of the Murchison Widefield Array (MWA), we calculate its sensitivity to the Epoch of Reionization (EoR) power spectrum of red- shifted 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, chro- matic 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 14σ detection of the EoR signal with one full season of observation on two fields (900 and 700 hours).

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arXiv2012

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.

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arXiv2012

A new layout optimization technique for interferometric arrays, applied to the MWA

Antenna layout is an important design consideration for radio interferometers because it determines the quality of the snapshot point spread function (PSF, or array beam). This is particularly true for experiments targeting the 21 cm Epoch of Reionization signal as the quality of the foreground subtraction depends directly on the spatial dynamic range and thus the smoothness of the baseline distribution. Nearly all sites have constraints on where antennas can be placed---even at the remote Australian location of the MWA (Murchison Widefield Array) there are rock outcrops, flood zones, heritages areas, emergency runways and trees. These exclusion areas can introduce spatial structure into the baseline distribution that enhance the PSF sidelobes and reduce the angular dynamic range. In this paper we present a new method of constrained antenna placement that reduces the spatial structure in the baseline distribution. This method not only outperforms random placement algorithms that avoid exclusion zones, but surprisingly outperforms random placement algorithms without constraints to provide what we believe are the smoothest constrained baseline distributions developed to date. We use our new algorithm to determine antenna placements for the originally planned MWA, and present the antenna locations, baseline distribution, and snapshot PSF for this array choice.

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arXiv2010

The Murchison Widefield Array

It is shown that the excellent Murchison Radio-astronomy Observatory site allows the Murchison Widefield Array to employ a simple RFI blanking scheme and still calibrate visibilities and form images in the FM radio band. The techniques described are running autonomously in our calibration and imaging software, which is currently being used to process an FM-band survey of the entire southern sky.

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arXiv2010

Alternative Adaptive Filter Structures for Improved Radio Frequency Interference Cancellation in Radio Astronomy

In radio astronomy, reference signals from auxiliary antennas that receive only the radio frequency interference (RFI) can be modified to model the RFI environment at the astronomy receivers. The RFI can then be canceled from the astronomy signal paths. However, astronomers typically only require signal statistics. If the RFI statistics are changing slowly, the cancellation can be applied to the signal correlations at a much lower rate than is required for standard adaptive filters. In this paper we describe five canceler setups; precorrelation and postcorrelation cancelers that use one or two reference signals in different ways. The theoretical residual RFI and added noise levels are examined and are demonstrated using microwave television RFI at the Australia Telescope Compact Array. The RFI is attenuated to below the system noise, a reduction of at least 20 dB. While dual-reference cancelers add more reference noise than single-reference cancelers, this noise is zero-mean and only adds to the system noise, decreasing the sensitivity. The residual RFI that remains in the output of single-reference cancelers (but not dual-reference cancelers) sets a nonzero noise floor that does not act like random system noise and may limit the achievable sensitivity. Thus, dual-reference cancelers often result in superior cancellation. Dual-reference precorrelation cancelers require a double-canceler setup to be useful and to give equivalent results to dual-reference postcorrelation cancelers.

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arXiv2009

The variability of Sagittarius A* at 3 millimeter

We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7*10^{-10}M_sun /yr is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*.

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arXiv2008

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 seconds. 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 instuments. Lessons from MWA pipeline development and processing of simulated data may be applied to future low-frequency fixed dipole arrays.

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arXiv2007

A Catalog of HI Clouds in the Large Magellanic Cloud

A 21 cm neutral hydrogen interferometric survey of the Large Magellanic Cloud (LMC) combined with the Parkes multi-beam HI single-dish survey clearly shows that the HI gas is distributed in the form of clumps or clouds. The HI clouds and clumps have been identified using a thresholding method with three separate brightness temperature thresholds ($T_b$). Each catalog of HI cloud candidates shows a power law relationship between the sizes and the velocity dispersions of the clouds roughly following the Larson Law scaling $σ_v \propto R^{0.5}$, with steeper indices associated with dynamically hot regions. The clouds in each catalog have roughly constant virial parameters as a function mass suggesting that that the clouds are all in roughly the same dynamical state, but the values of the virial parameter are significantly larger than unity showing that turbulent motions dominate gravity in these clouds. The mass distribution of the clouds is a power law with differential indices between -1.6 and -2.0 for the three catalogs. In contrast, the distribution of mean surface densities is a log-normal distribution.

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arXiv2007

Imaging algorithms in radio interferometry

The paper reviews progress in imaging in radio interferometry for the period 1993-1996. Unlike an optical telescope, the basic measurements of a radio interferometer (correlations between antennas) are indirectly related to a sky brightness image. In a real sense, algorithms and computers are the lenses of a radio interferometer. In the last 20 years, whereas interferometer hardware advances have resulted in improvements of a factor of a few, algorithm and computer advances have resulted in orders of magnitude improvement in image quality. Developing these algorithms has been a fruitful and comparatively inexpensive method of improving the performance of existing telescopes, and has made some newer telescopes possible. In this paper, we review recent developments in the algorithms used in the imaging part of the reduction process. What constitutes an `imaging algorithm'? Whereas once there was a steady `forward' progression in the reduction process of editing, calibrating, transforming and, finally, deconvolving, this is no longer true. The introduction of techniques such as self-calibration, and algorithms that go directly from visibilities to final images, have made the dividing lines less clear. Although we briefly consider self-calibration, for the purposes of this paper calibration issues are generally excluded. Most attention will be directed to the steps which form final images from the calibrated visibilities.

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arXiv2006

Radio path length correction using water vapour radiometry

Path length changes through the atmosphere cause significant errors for astronomical radio interferometry at high frequencies (e.g. 100 GHz). Path length differences typically give rise to a differential excess path of 1mm for antennas separated by 1km, and have fluctuation time scales of greater than 10 seconds. To measure these fluctuations, we are building a four-channel radiometer centred on the 22 GHz water line. By sensing the water vapour emission, the excess path can be deduced and corrected. Multiple channels give us robustness against various systematic errors, but gain stability of the radiometer of 1 pair in 10^4 is still required.

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arXiv2006

A retrospective view of Miriad

Miriad is a radio interferometry data-reduction package, designed for taking raw data through to the image analysis stage. The Miriad project, begun in 1988, is now middle-aged. With the wisdom of hindsight, we review design decisions and some of Miriad's characteristics.

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arXiv2006

Longitude-resolved imaging of Jupiter at lambda = 2 cm

We present a technique for creating a longitude-resolved image of Jupiter's thermal radio emission. The technique has been applied to VLA data taken on 25 January 1996 at a wavelength of 2 cm. A comparison with infrared data shows a good correlation between radio hot spots and the 5 micron hot spots seen on IRTF images. The brightest spot on the radio image is most likely the hot spot through which the Galileo probe entered Jupiter's atmosphere. We derived the ammonia abundance (= volume mixing ratio) in the hot spot, which is ~ 3 x 10^{-5}, about half that seen in longitude-averaged images of the NEB, or less than 1/3 of the longitude-averaged ammonia abundance in the EZ. This low ammonia abundance probably extends down to at least the 4 bar level.

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arXiv2006

1.4 GHz polarimetric observations of the two fields imaged by the DASI experiment

We present results of polarization observations at 1.4 GHz of the two fields imaged by the DASI experiment ($α= 23^{\rm h} 30^{\rm m}$, $δ= -55^{\circ}$ and $α= 00^{\rm h} 30^{\rm m}$, $δ= -55^{\circ}$, respectively). Data were taken with the Australia Telescope Compact Array with 3.4 arcmin resolution and $\sim 0.18$ mJy beam$^{-1}$ sensitivity. The emission is dominated by point sources and we do not find evidence for diffuse synchrotron radiation even after source subtraction. This allows to estimate an upper limit of the diffuse polarized emission. The extrapolation to 30 GHz suggests that the synchrotron radiation is lower than the polarized signal measured by the DASI experiment by at least 2 orders of magnitude. This further supports the conclusions drawn by the DASI team itself about the negligible Galactic foreground contamination in their data set, improving by a factor $\sim 5$ the upper limit estimated by Leitch et al. (2005). The dominant point source emission allows us to estimate the contamination of the CMB by extragalactic foregrounds. We computed the power spectrum of their contribution and its extrapolation to 30 GHz provides a framework where the CMB signal should dominate. However, our results do not match the conclusions of the DASI team about the negligibility of point source contamination, suggesting to take into account a source subtraction from the DASI data.

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arXiv2005

High-frequency radio observations of the Kuehr sample and the epoch-dependent luminosity function of the flat-spectrum quasars

We discuss our ATCA 18.5 and 22 GHz flux density measurements of the Southern extragalactic sources in the complete 5-GHz sample of Kuehr et al. (1981). The high-frequency (5-18.5 GHz) spectral indices of steep-spectrum sources for which we have 18.5-GHz data (66% of the complete sample) are systematically steeper than the low-frequency (2.7-5 GHz) ones, and there is evidence of an anti-correlation of high-frequency spectral index with luminosity. The completeness of the 18.5-GHz data is much higher (89%) for flat-spectrum sources (mostly quasars), which also exhibit a spectral steepening. Taking advantage of the almost complete redshift information on flat-spectrum quasars, we have estimated their 5-GHz luminosity function in several redshift bins. The results confirm that their radio luminosity density peaks at z_peak \simeq 2.5 but do not provide evidence for deviations from pure luminosity evolution as hinted at by other data sets. A comparison of our 22-GHz flux density with WMAP K-band data for flat-spectrum sources suggests that WMAP flux densities may be low by a median factor of \simeq 1.2. The extrapolations of 5-GHz counts and luminosity functions of flat-spectrum radio quasars using the observed distribution of the 5-18.5 GHz spectral indices match those derived directly from WMAP data, indicating that the high-frequency WMAP survey does not detect any large population of FSRQs with anomalous spectra.

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arXiv2005

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

Using a Fourier-plane technique, we have merged HI ATCA and Parkes data sets providing an accurate set of images of the LMC sensitive to structure on scales of 15 pc (for an LMC distance of 55 kpc) upwards. The spatial dynamic range (2.8 orders of magnitude), velocity resolution (1.649 km/s), brightness temperature sensitivity (2.4 K) and column density sensitivity ($8.9 \times 10^ {18}$ cm$^{-2}$ per 1.649 km/s channel) allow for the studies of phenomena ranging from the galaxy-wide interaction of the LMC with its close neighbours to the small-scale injection of energy from supernovae and stellar associations into the ISM of the LMC. We find that a previous analysis of the holes and shells (Kim et al. 1999) in the HI distribution shows the same power- law behavior with that of clumps. The measured perimeter P and measured enclosed area A of each HI cloud in a log-log plot give us a set of points lying along with a slope of D/2 = 0.73 $\pm$ 0.1. The relation between area and perimeter of each identified HI cloud, P $\propto$ $A^{D/2}$, can determine the fractal dimension D=1.47 $\pm$ 0.2 of a cloud boundary (Vogelaar & Wakker 1994; Williams, Blitz, & McKee 2000). The measured fractal dimension of HI clouds in the LMC is similar to the dimension, D=1.4, found in many studies of the molecular ISM.

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