arXiv subjects

Steven Tingay

Publications and source records attributed to Steven Tingay.

At least 37 records · Page 2Linked to original sources

arXiv2018

Interplanetary Scintillation studies with the Murchison Wide-field Array IV: The hosts of sub-arcsecond compact sources at low radio frequencies

Around 10% of bright low-frequency radio sources observed with the Murchison Widefield Array (MWA) show strong Interplanetary Scintillation (IPS) on timescales of a few seconds, implying that almost all their low-frequency radio emission comes from a compact component less than 0.5 arcsec in angular size. Most of these objects are compact steep-spectrum (CSS) or MHz-peaked spectrum (MPS) radio sources. We have used mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) catalogue to search for the host galaxies of 65 strongly-scintillating MWA sources and compare their properties with those of the overall population of bright low-frequency radio sources. We identified WISE mid-infrared counterparts for 91% of the bright sources in a single 900 square degree MWA field, and found that the hosts of the strongly-scintillating sources were typically at least 1 mag fainter in the WISE W1 (3.4 micron) band than the hosts of weakly-scintillating MWA sources of similar radio flux density. This difference arises mainly because the strongly-scintillating sources are more distant. We estimate that strongly-scintillating MWA sources have a median redshift of z ~ 1.5, and that at least 30% of them are likely to lie at z > 2. The recently-developed wide-field IPS technique therefore has the potential to provide a powerful new tool for identifying high-redshift radio galaxies without the need for radio spectral-index selection.

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arXiv2017

Ultimate precision in cosmic-ray radio detection --- the SKA

As of 2023, the low-frequency part of the Square Kilometre Array will go online in Australia. It will constitute the largest and most powerful low-frequency radio-astronomical observatory to date, and will facilitate a rich science programme in astronomy and astrophysics. With modest engineering changes, it will also be able to measure cosmic rays via the radio emission from extensive air showers. The extreme antenna density and the homogeneous coverage provided by more than 60,000 antennas within an area of one km$^2$ will push radio detection of cosmic rays in the energy range around 10$^{17}$ eV to ultimate precision, with superior capabilities in the reconstruction of arrival direction, energy, and an expected depth-of-shower-maximum resolution of 6~g/cm${^2}$.

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arXiv2016

A search for long-timescale, low-frequency radio transients

We present a search for transient and highly variable sources at low radio frequencies (150-200 MHz) that explores long timescales of 1-3 years. We conducted this search by comparing the TIFR GMRT Sky Survey Alternative Data Release 1 (TGSS ADR1) and the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey catalogues. To account for the different completeness thresholds in the individual surveys, we searched for compact GLEAM sources above a flux density limit of 100 mJy that were not present in the TGSS ADR1; and also for compact TGSS ADR1 sources above a flux density limit of 200 mJy that had no counterpart in GLEAM. From a total sample of 234 333 GLEAM sources and 275 612 TGSS ADR1 sources in the overlap region between the two surveys, there were 99658 GLEAM sources and 38 978 TGSS ADR sources that passed our flux density cutoff and compactness criteria. Analysis of these sources resulted in three candidate transient sources. Further analysis ruled out two candidates as imaging artefacts. We analyse the third candidate and show it is likely to be real, with a flux density of 182 +/- 26 mJy at 147.5 MHz. This gives a transient surface density of rho = (6.2 +/- 6) x 10-5 deg-2 . We present initial follow-up observations and discuss possible causes for this candidate. The small number of spurious sources from this search demonstrates the high reliability of these two new low-frequency radio catalogues.

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arXiv2016

Absorption variability as a probe of the multiphase interstellar media surrounding active galaxies

We examine a model for the variable free-free and neutral hydrogen absorption inferred towards the cores of some compact radio galaxies in which a spatially fluctuating medium drifts in front of the source. We relate the absorption-induced intensity fluctuations to the statistics of the underlying opacity fluctuations. We investigate models in which the absorbing medium consists of either discrete clouds or a power-law spectrum of opacity fluctuations. We examine the variability characteristics of a medium comprised of Gaussian-shaped clouds in which the neutral and ionized matter are co-located, and in which the clouds comprise spherical constant-density neutral cores enveloped by ionized sheaths. The cross-power spectrum indicates the spatial relationship between neutral and ionized matter, and distinguishes the two models, with power in the Gaussian model declining as a featureless power-law, but that in the ionized sheath model oscillating between positive and negative values. We show how comparison of the HI and free-free power spectra reveals information on the ionization and neutral fractions of the medium. The background source acts as a low-pass filter of the underlying opacity power spectrum, which limits temporal fluctuations to frequencies $ω< \dotθ_v / θ_{\rm src}$, where $\dotθ_v$ is the angular drift speed of the matter in front of the source, and it quenches the observability of opacity structures on scales smaller than the source size $θ_{\rm src}$. For drift speeds of $\sim 10^3\,$km s$^{-1}$ and source brightness temperatures $\sim 10^{12}\,$K, this limitation confines temporal opacity fluctuations to timescales of order several months to decades.

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arXiv2014

Very Long Baseline Interferometry with the SKA

Adding VLBI capability to the SKA arrays will greatly broaden the science of the SKA, and is feasible within the current specifications. SKA-VLBI can be initially implemented by providing phased-array outputs for SKA1-MID and SKA1-SUR and using these extremely sensitive stations with other radio telescopes, and in SKA2 by realising a distributed configuration providing baselines up to thousands of km, merging it with existing VLBI networks. The motivation for and the possible realization of SKA-VLBI is described in this paper.

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arXiv2014

Serendipitous discovery of a dying Giant Radio Galaxy associated with NGC 1534, using the Murchison Widefield Array

Recent observations with the Murchison Widefield Array at 185~MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby ($z = 0.0178$) radio galaxy associated with NGC\,1534. The diffuse emission presented here is the first indication that NGC\,1534 is one of a rare class of objects (along with NGC\,5128 and NGC\,612) in which a galaxy with a prominent dust lane hosts radio emission on scales of $\sim$700\,kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with disks. NGC1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2\,mJy\,arcmin$^{-2}$. The radio lobes have one of the steepest spectral indices yet observed: $α=-2.1\pm0.1$, and the core to lobe luminosity ratio is $<0.1$\%. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as $7\times10^{-7}$\,Mpc$^{-3}$ and argue that normal AGN cannot spend more than 6\% of their lifetime in this phase if they all go through the same cycle.

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arXiv2014

The Murchison Widefield Array Commissioning Survey: A Low-Frequency Catalogue of 14,110 Compact Radio Sources over 6,100 Square Degrees

We present the results of an approximately 6,100 square degree 104--196MHz radio sky survey performed with the Murchison Widefield Array during instrument commissioning between 2012 September and 2012 December: the Murchison Widefield Array Commissioning Survey (MWACS). The data were taken as meridian drift scans with two different 32-antenna sub-arrays that were available during the commissioning period. The survey covers approximately 20.5 h < Right Ascension (RA) < 8.5 h, -58 deg < Declination (Dec) < -14 deg over three frequency bands centred on 119, 150 and 180 MHz, with image resolutions of 6--3 arcmin. The catalogue has 3-arcmin angular resolution and a typical noise level of 40 mJy/beam, with reduced sensitivity near the field boundaries and bright sources. We describe the data reduction strategy, based upon mosaiced snapshots, flux density calibration and source-finding method. We present a catalogue of flux density and spectral index measurements for 14,110 sources, extracted from the mosaic, 1,247 of which are sub-components of complexes of sources.

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arXiv2012

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 widefield 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.

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arXiv2011

First geodetic observations using new VLBI stations ASKAP-29 and WARK12M

We report the results of a successful 7 hour 1.4 GHz VLBI experiment using two new stations, ASKAP-29 located in Western Australia and WARK12M located on the North Island of New Zealand. This was the first geodetic VLBI observing session with the participation of these new stations. We have determined the positions of ASKAP-29 and WARK12M. Random errors on position estimates are 150-200 mm for the vertical component and 40-50 mm for the horizontal component. Systematic errors caused by the unmodeled ionosphere path delay may reach 1.3 m for the vertical component.

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arXiv2010

Wide-field VLBI imaging of M31 - first results

One of our closest neighbours, the Andromeda Galaxy (M31) has been the subject of numerous large area studies across the entire spectrum, but so far full-disk radio surveys have been conducted only at low resolution. The new wide-field capabilities of the DiFX software correlator present the possibility of imaging the entire primary beam of a VLBI array, thus enabling a high resolution wide-field study of the entire galaxy. Using the VLBA and EVN, pilot observations of M31 have been carried out with the aim of using these new wide-field techniques to characterise the population of compact components at VLBI resolution both within and behind one of our nearest neighbours. This contribution describes the observations carried out, the preliminary processing and first results.

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arXiv2010

Finding AGN with wide-field VLBI observations

VLBI observations are a reliable method to identify AGN, since they require high brightness temperatures for a detection to be made. However, because of the tiny fields of view it is unpractical to carry out VLBI observations of many sources using conventional methods. We used an extension of the DiFX software correlator to image with high sensitivity 96 sources in the Chandra Deep Field South, using only 9h of observing time with the VLBA. We detected 20 sources, 8 of which had not been identified as AGN at any other wavelength, despite the comprehensive coverage of this field. The lack of X-ray counterparts to 1/3 of the VLBI-detected sources, despite the sensitivity of co-located X-ray data, demonstrates that X-ray observations cannot be solely relied upon when searching for AGN activity. Surprisingly, we find that sources classified as type 1 QSOs using X-ray data are always detected, in contrast to the 10% radio-loud objects which are found in optically-selected QSOs. We present the continuation of this project with the goal to image 1450 sources in the Lockman Hole/XMM region.

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arXiv2010

Wide-field VLBA Observations of the Chandra Deep Field South

Wide-field surveys are a commonly-used method for studying thousands of objects simultaneously, to investigate, e.g., the joint evolution of star-forming galaxies and active galactic nuclei. VLBI observations can yield valuable input to such studies because they are able to identify AGN. However, VLBI observations of large swaths of the sky are impractical using standard methods, because the fields of view of VLBI observations are of the order of 10" or less. We have embarked on a project to carry out Very Long Baseline Array (VLBA) observations of all 96 known radio sources in one of the best-studied areas in the sky, the Chandra Deep Field South (CDFS). The challenge was to develop methods which could significantly reduce the amount of observing (and post-processing) time. We have developed an extension to the DiFX software correlator which allows one to correlate hundreds of positions within the primary beams. This extension enabled us to target many sources, at full resolution and high sensitivity, using only a small amount of observing time. The combination of wide fields-of-view and high sensitivity across the field in this survey is unprecedented. We have observed with the VLBA a single pointing containing the Chandra Deep Field South, in which 96 radio sources were known from previous observations with the ATCA. From our input sample, 20 were detected with the VLBA. The majority of objects have flux densities in agreement with arcsec-scale observations, implying that their radio emission comes from very small regions. One VLBI-detected object had earlier been classified as a star-forming galaxy. Comparing the VLBI detections to sources found in sensitive, co-located X-ray observations we find that X-ray detections are not a good indicator for VLBI detections. Wide-field VLBI survey science is now coming of age.

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arXiv2009

100 Microarcsecond Resolution VLBI Imaging of Anisotropic Interstellar Scattering towards Pulsar B0834+06

We have invented a novel technique to measure the radio image of a pulsar scattered by the interstellar plasma with 0.1 mas resolution. We extend the "secondary spectrum" analysis of parabolic arcs by Stinebring et al. (2001) to very long baseline interferometry and, when the scattering is anisotropic, we are able to map the scattered brightness astrometrically with much higher resolution than the diffractive limit of the interferometer. We employ this technique to measure an extremely anisotropic scattered image of the pulsar B0834+06 at 327 MHz. We find that the scattering occurs in a compact region about 420 pc from the Earth. This image has two components, both essentially linear and nearly parallel. The primary feature, which is about 16 AU long and less than 0.5 AU in width, is highly inhomogeneous on spatial scales as small as 0.05 AU. The second feature is much fainter and is displaced from the axis of the primary feature by about 9 AU. We find that the velocity of the scattering plasma is 16+-10 km/s approximately parallel to the axis of the linear feature. The origin of the observed anisotropy is unclear and we discuss two very different models. It could be, as has been assumed in earlier work, that the turbulence on spatial scales of ~1000 km is homogeneous but anisotropic. However it may be that the turbulence on these scales is homogeneous and isotropic but the anisotropy is produced by highly elongated (filamentary) inhomogeneities of scale 0.05-16 AU.

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arXiv2009

The Murchison Widefield Array: Design Overview

The Murchison Widefield Array (MWA) 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. These are detection and characterization of 3-dimensional brightness temperature fluctuations in the 21cm line of neutral hydrogen during the Epoch of Reionization (EoR) at redshifts from 6 to 10, 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 broad-band active dipoles, arranged into 512 tiles comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5km in diameter, with a small number of outliers extending to 3km. All tile-tile baselines are correlated in custom FPGA-based hardware, yielding a Nyquist-sampled instantaneous monochromatic uv coverage and unprecedented point spread function (PSF) 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.

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arXiv2008

The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705

In this article, we demonstrate that PKSJ 1037-2705 has a weak accretion flow luminosity, well below the Seyfert1/QSO dividing line, weak broad emission lines (BELs) and moderately powerful FRII extended radio emission. It is one of the few documented examples of a broad-line object in which the time averaged jet kinetic luminosity, $\bar{Q}$, is larger than the total thermal luminosity (IR to X-ray) of the accretion flow, $L_{bol}$. The blazar nucleus dominates the optical and near ultraviolet emission and is a strong source of hard X-rays. The strong blazar emission indicates that the relativistic radio jet is presently active. The implication is that even weakly accreting AGN can create powerful jets. Kinetically dominated ($\bar{Q}>L_{bol}$) broad-line objects provide important constraints on the relationship between the accretion flow and the jet production mechanism.

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arXiv2007

VLBI detection of an Infrared-Faint Radio Source

Infrared-Faint Radio Sources represent a new and unexpected class of object which is bright at radio wavelengths but unusually faint at infrared wavelengths. If, like most mJy radio sources, they were either conventional active or star-forming galaxies in the local Universe, we would expect them to be detectable at infrared wavelengths, and so their non-detection by the Spitzer Space Telescope is surprising. Here we report the detection of one of these sources using Very Long Baseline Interferometry, from which we conclude that the sources are driven by Active Galactic Nuclei. We suggest that these sources are either normal radio-loud quasars at high redshift or abnormally obscured radio galaxies.

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arXiv2006

PKS 1018-42: A Powerful Kinetically Dominated Quasar

We have identified PKS 1018-42 as a radio galaxy with extraordinarily powerful jets, over twice as powerful as any 3CR source of equal or lesser redshift except for one (3C196). It is perhaps the most intrinsically powerful extragalactic radio source in the, still poorly explored, Southern Hemisphere. PKS 1018-42 belongs to the class of FR II objects that are kinetically dominated, the jet kinetic luminosity, $Q \sim 6.5 \times 10^{46}\mathrm{ergs/s}$ (calculated at 151 MHz), is 3.4 times larger than the total thermal luminosity (IR to X-ray) of the accretion flow, $L_{bol} \sim 1.9 \times 10^{46}\mathrm{ergs/s}$. It is the fourth most kinetically dominated quasar that we could verify from existing radio data. From a review of the literature, we find that kinetically dominated sources such as PKS 1018-42 are rare, and list the 5 most kinetically dominated sources found from our review. Our results for PKS 1018-42 are based on new observations from the Australia Telescope Compact Array.

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arXiv2005

PKS 0743$-$67: An Ultra-luminous Accretion Disk and a High Kinetic Luminosity Jet

In this letter, deep radio observations of the quasar PKS 0743$-$67 are presented that reveal a central engine capable of driving jets with enormous kinetic luminosity, $Q>4.1 \times 10^{46}\mathrm{ergs/s}$. This result is significant because archival optical spectral data indicates that the accretion disk has a thermal luminosity, $L_{bol}>2\times 10^{47}\mathrm{ergs/s}$. Furthermore, estimates of the central black hole mass from line widths indicate that $L_{bol}/L_{Edd}\approx 1$. This suggests that neither a large $L_{bol}$ nor $L_{bol}/L_{Edd}$ suppresses jet power in quasars, despite claims that they do in the recent literature. Earlier studies have found $L_{bol}$ and $Q$ are correlated in blazars. However, by removing the BL-Lacs and leaving only the quasars in the sample, we found that $Q$ is very weakly correlated with $L_{bol}$ in the subsample.

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