arXiv subjects

R. Ekers

Publications and source records attributed to R. Ekers.

At least 19 recordsLinked to original sources

arXiv2025

Relevance of Murchison Widefield Array Interplanetary Scintillation Observations to Heliospheric Transient Catalogues

We have conducted a comprehensive comparison of interplanetary scintillation (IPS) observations taken by the Murchison Widefield Array (MWA) with several heliospheric transient event catalogues, over a time period of 7 months during solar minimum. From this analysis we have found that of the 84% of catalogued events that have MWA IPS data available, 68% of them appear in MWA observations. Of the enhancements first identified in IPS observations, only 58% have a potential match with a catalogued event. The majority of enhancements that were identified in the IPS observations were situated greater than 10$^\circ$ from the ecliptic plane. Two such features were selected for detailed analysis, connecting their solar origins to their propagation through the heliosphere. The first of these features was created by a coronal mass ejection (CME), captured over two successive MWA observations and recorded in several catalogues. The second feature has the potential of being a stream interaction region (SIR) travelling out of the ecliptic plane. This particular SIR was not recorded in any catalogue. Thus the MWA shows promise in detecting heliospheric transients that other commonly-used techniques may overlook. These results show the strength of the MWA in having unbridled access to the heliosphere, able to make remote observations of events far out of the ecliptic as it is not restrained to the orbits of spacecraft. We demonstrate how the inclusion of MWA IPS data can potentially boost the number of CME and SIR events that are characterised.

astro-ph.SR↗

arXiv2025

Latitudinal dependence of the solar wind during periods of high and low activity through interplanetary scintillation

We present a study of the solar wind over different periods of the solar cycle, specifically focussing on the minimum between solar cycles 24 and 25, and the active, ascending phase of solar cycle 25. With the use of interplanetary scintillation (IPS) data taken by the Murchison Widefield Array (MWA) from mid-2019 and early 2023, we have sampled over a wide range of solar latitudes and elongations, probing a large section of the surrounding heliosphere from $\sim$90 to 140R$_\odot$. The MWA observations provide the highest density of sampled IPS radio sources to date, allowing for an investigation into the latitudinal dependence of the scattering effect caused by the solar wind on a radio source as observed throughout the solar cycle. We find our measurements during periods of heightened solar activity are consistent with a spherically symmetric solar wind. On the other hand, with a reduction in solar activity we find the solar wind density inherits a latitudinal dependence. As is consistent with prior studies, an elliptical function better represents the transition from poles to equator, although we find a more exaggerated sigmoid shaped curve is required to represent the low- to mid-latitude transition region during the minimum of solar cycle 24. We find for a heliospheric distance range of 108 - 123R$_{\odot}$ the reduction ratio between the equator and the southern pole is 1.62$\pm$0.02.

astro-ph.SR↗

arXiv2023

Resolving moving heliospheric structures using interplanetary scintillation observations with the Murchison Widefield Array

We have conducted a blind search in 49 consecutive days of interplanetary scintillation observations made by the Murchison Widefield Array from mid-2019, with overlapping daily observations approximately East and South-East of the Sun at an elongation of $\sim$30 degrees and a field of view of 30 degrees. These observations detect an unprecedented density of sources. In spite of these observations being taken at sunspot minimum, this search has revealed several interesting transitory features characterised by elevated scintillation levels. One solar wind enhancement is captured in two observations several hours apart, allowing its radial movement away from the Sun to be measured. We present here a methodology for measuring the plane-of-sky velocity for the moving heliospheric structure. The plane-of-sky velocity was inferred as $0.66\pm0.147\,^{\text{o}}\text{hr}^{-1}$, or $480\pm106\,\text{km}\,\text{s}^{-1}$ assuming a distance of 1AU. After cross-referencing our observed structure with multiple catalogues of heliospheric events, we propose that the likely source of our observed structure is a stream-interaction region originating from a low-latitude coronal hole. This work demonstrates the power of widefield interplanetary scintillation observations to capture detailed features in the heliosphere which are otherwise unresolvable and go undetected.

astro-ph.SR↗

arXiv2023

Detection and Characterisation of a Coronal Mass Ejection using Interplanetary Scintillation measurements from the Murchison Widefield Array

We have shown previously that the Murchison Widefield Array (MWA), can detect hundreds of Interplanetary Scintillation (IPS) sources simultaneously across a field of view $\sim30^\circ$ in extent. To test if we can use this capability to track heliospheric structures, we undertook a search of 88 hours of MWA IPS data, and identified an observation likely to have a significant Coronal Mass Ejection (CME) in the field of view. We demonstrate that in a single 5-minute MWA observation we are able to localise and image a CME $\sim$33 hours after launch at an elongation of $\sim37^\circ$ from the Sun. We use IPS observables to constrain the kinematics of the CME, and describe how MWA IPS observations can be used in the future to make a unique contribution to heliospheric modelling efforts.

astro-ph.SR↗

arXiv2017

The jet/wind outflow in Centaurus A: a local laboratory for AGN feedback

We present new radio and optical images of the nearest radio galaxy Centaurus A and its host galaxy NGC 5128. We focus our investigation on the northern transition region, where energy is transported from the ~5 kpc (~5 arcmin) scales of the Northern Inner Lobe (NIL) to the ~30 kpc (~30 arcmin) scales of the Northern Middle Lobe (NML). Our Murchison Widefield Array observations at 154 MHz and our Parkes radio telescope observations at 2.3 GHz show diffuse radio emission connecting the NIL to the NML, in agreement with previous Australia Telescope Compact Array observations at 1.4 GHz. Comparison of these radio data with our widefield optical emission line images show the relationship between the NML radio emission and the ionised filaments that extend north from the NIL, and reveal a new ionised filament to the east, possibly associated with a galactic wind. Our deep optical images show clear evidence for a bipolar outflow from the central galaxy extending to intermediate scales, despite the non-detection of a southern radio counterpart to the NML. Thus, our observational overview of Centaurus A reveals a number of features proposed to be associated with AGN feedback mechanisms, often cited as likely to have significant effects in galaxy evolution models. As one of the closest galaxies to us, Centaurus A therefore provides a unique laboratory to examine feedback mechanisms in detail.

astro-ph.GA↗

arXiv2017

Interplanetary Scintillation with the Murchison Widefield Array I: A sub-arcsecond Survey over 900 square degrees at 79 and 158 MHz

We present the first dedicated observations of Interplanetary Scintillation (IPS) with the Murchison Widefield Array (MWA). We have developed a synthesis imaging technique, tailored to the properties of modern "large-N" low-frequency radio telescopes. This allows us to image the variability on IPS timescales across 900 square degrees simultaneously. We show that for our observations, a sampling rate of just 2Hz is sufficient to resolve the IPS signature of most sources. We develop tests to ensure that IPS variability is separated from ionospheric or instrumental variability. We validate our results by comparison with existing catalogues of IPS sources, and near-contemporaneous observations by other IPS facilities. Using just five minutes of data, we produce catalogues at both 79MHz and 158MHz, each containing over 350 scintillating sources. At the field centre we detect approximately one scintillating source per square degree, with a minimum scintillating flux density at 158MHz of 110mJy, corresponding to a compact flux density of approximately 400mJy. Each of these sources is a known radio source, however only a minority were previously known to contain sub-arcsecond components. We discuss our findings and the prospects they hold for future astrophysical and heliospheric studies.

astro-ph.IM↗

arXiv2015

A pilot ASKAP survey of radio transient events in the region around the intermittent pulsar PSR J1107-5907

We use observations from the Boolardy Engineering Test Array (BETA) of the Australian Square Kilometre Array Pathfinder (ASKAP) telescope to search for transient radio sources in the field around the intermittent pulsar PSR J1107-5907. The pulsar is thought to switch between an "off" state in which no emission is detectable, a weak state and a strong state. We ran three independent transient detection pipelines on two-minute snapshot images from a 13 hour BETA observation in order to 1) study the emission from the pulsar, 2) search for other transient emission from elsewhere in the image and 3) to compare the results from the different transient detection pipelines. The pulsar was easily detected as a transient source and, over the course of the observations, it switched into the strong state three times giving a typical timescale between the strong emission states of 3.7 hours. After the first switch it remained in the strong state for almost 40 minutes. The other strong states lasted less than 4 minutes. The second state change was confirmed using observations with the Parkes radio telescope. No other transient events were found and we place constraints on the surface density of such events on these timescales. The high sensitivity Parkes observations enabled us to detect individual bright pulses during the weak state and to study the strong state over a wide observing band. We conclude by showing that future transient surveys with ASKAP will have the potential to probe the intermittent pulsar population.

astro-ph.SR↗

arXiv2015

Discovery of HI gas in a young radio galaxy at $z = 0.44$ using the Australian Square Kilometre Array Pathfinder

We report the discovery of a new 21-cm HI absorption system using commissioning data from the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP). Using the 711.5 - 1015.5 MHz band of ASKAP we were able to conduct a blind search for the 21-cm line in a continuous redshift range between $z = 0.4$ and 1.0, which has, until now, remained largely unexplored. The absorption line is detected at $z = 0.44$ towards the GHz-peaked spectrum radio source PKS B1740$-$517 and demonstrates ASKAP's excellent capability for performing a future wide-field survey for HI absorption at these redshifts. Optical spectroscopy and imaging using the Gemini-South telescope indicates that the HI gas is intrinsic to the host galaxy of the radio source. The narrow OIII emission lines show clear double-peaked structure, indicating either large-scale outflow or rotation of the ionized gas. Archival data from the \emph{XMM-Newton} satellite exhibit an absorbed X-ray spectrum that is consistent with a high column density obscuring medium around the active galactic nucleus. The HI absorption profile is complex, with four distinct components ranging in width from 5 to 300 km s$^{-1}$ and fractional depths from 0.2 to 20 per cent. In addition to systemic HI gas, in a circumnuclear disc or ring structure aligned with the radio jet, we find evidence for a possible broad outflow of neutral gas moving at a radial velocity of $v \sim 300$ km s$^{-1}$. We infer that the expanding young radio source ($t_{\rm age} \approx 2500$ yr) is cocooned within a dense medium and may be driving circumnuclear neutral gas in an outflow of $\sim$ 1 $\mathrm{M}_{\odot}$ yr$^{-1}$.

astro-ph.GA↗

arXiv2014

The Australian Square Kilometre Array Pathfinder: System Architecture and Specifications of the Boolardy Engineering Test Array

This paper describes the system architecture of a newly constructed radio telescope - the Boolardy Engineering Test Array, which is a prototype of the Australian Square Kilometre Array Pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a 6-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least 9 dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations.

astro-ph.IM↗

arXiv2012

Periodic structure in the Mpc-scale jet of PKS 0637-752

We present 18 GHz Australia Telescope Compact Array imaging of the Mpc-scale quasar jet PKS 0637-752 with angular resolution ~0.58 arcseconds. We draw attention to a spectacular train of quasi-periodic knots along the inner 11 arcseconds of the jet, with average separation ~1.1 arcsec (7.6 kpc projected). We consider two classes of model to explain the periodic knots: those that involve a static pattern through which the jet plasma travels (e.g. stationary shocks); and those that involve modulation of the jet engine. Interpreting the knots as re-confinement shocks implies the jet kinetic power Q ~ 10^{46} erg/s, but the constant knot separation along the jet is not expected in a realistic external density profile. For models involving modulation of the jet engine, we find that the required modulation period is 2 x 10^3 yr < τ< 3 x 10^5 yr. The lower end of this range is applicable if the jet remains highly relativistic on kpc-scales, as implied by the IC/CMB model of jet X-ray emission. We suggest that the quasi-periodic jet structure in PKS 0637-752 may be analogous to the quasi-periodic jet modulation seen in the microquasar GRS 1915+105, believed to result from limit cycle behaviour in an unstable accretion disk. If variations in the accretion rate are driven by a binary black hole, the predicted orbital radius is 0.7 < a < 30 pc, which corresponds to a maximum angular separation of ~0.1 - 5 mas.

astro-ph.CO↗

arXiv2011

Very High Angular Resolution Science with the Square Kilometre Array

Preliminary specifications for the Square Kilometre Array (SKA) call for 25% of the total collecting area of the dish array to be located at distances greater than 180 km from the core, with a maximum baseline of at least 3000 km. The array will provide angular resolution ~ 40 - 2 mas at 0.5 - 10 GHz with image sensitivity reaching < 50 nJy/beam in an 8 hour integration with 500 MHz bandwidth. Given these specifications, the high angular resolution component of the SKA will be capable of detecting brightness temperatures < 200 K with milliarcsecond-scale angular resolution. The aim of this article is to bring together in one place a discussion of the broad range of new and important high angular resolution science that will be enabled by the SKA, and in doing so, address the merits of long baselines as part of the SKA. We highlight the fact that high angular resolution requiring baselines greater than 1000 km provides a rich science case with projects from many areas of astrophysics, including important contributions to key SKA science.

astro-ph.IM↗

arXiv2011

A new method to calibrate ionospheric pulse dispersion for UHE cosmic ray and neutrino detection using the Lunar Cherenkov technique

UHE particle detection using the lunar Cherenkov technique aims to detect nanosecond pulses of Cherenkov emission which are produced during UHE cosmic ray and neutrino interactions in the Moon's regolith. These pulses will reach Earth-based telescopes dispersed, and therefore reduced in amplitude, due to their propagation through the Earth's ionosphere. To maximise the received signal to noise ratio and subsequent chances of pulse detection, ionospheric dispersion must therefore be corrected, and since the high time resolution would require excessive data storage this correction must be made in real time. This requires an accurate knowledge of the dispersion characteristic which is parameterised by the instantaneous Total Electron Content (TEC) of the ionosphere. A new method to calibrate the dispersive effect of the ionosphere on lunar Cherenkov pulses has been developed for the LUNASKA lunar Cherenkov experiments. This method exploits radial symmetries in the distribution of the Moon's polarised emission to make Faraday rotation measurements in the visibility domain of synthesis array data (i. e. instantaneously). Faraday rotation measurements are then combined with geomagnetic field models to estimate the ionospheric TEC. This method of ionospheric calibration is particularly attractive for the lunar Cherenkov technique as it may be used in real time to estimate the ionospheric TEC along a line-of-sight to the Moon and using the same radio telescope.

astro-ph.HE↗

arXiv2008

Science with ASKAP - the Australian Square Kilometre Array Pathfinder

[ABRIDGED VERSION] The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised to achieve substantial advances in SKA key science. The central core of ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of the sites selected by the international community as a potential location for the SKA. Following an introductory description of ASKAP, this document contains 7 chapters describing specific science programmes for ASKAP. The combination of location, technological innovation and scientific program will ensure that ASKAP will be a world-leading radio astronomy facility, closely aligned with the scientific and technical direction of the SKA. A brief summary chapter emphasizes the point, and considers discovery space.

astro-ph↗

arXiv2006

Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies

Aims: We present new radio continuum observations of ten BIMA SONG galaxies, taken at 1.4 GHz with the Very Large Array. These observations allow us to extend the study of the relationships between the radio continuum (RC) and CO emission to 22 CO luminous galaxies for which single dish CO images have been added to interferometric data. New Spitzer infrared (IR) images of six of these galaxies have been released. The analysis of these high resolution images allowed us to probe the RC-IR-CO correlations down to linear scales of a few hundred pc. Results: for the 22 galaxies analysed, the RC-CO correlation on scales from $\sim10$ kpc down to $\sim100$ pc is nearly linear and has a scatter of a factor of two, i.e. comparable to that of the global correlations. There is no evidence for any severe degradation of the scatter below the kpc scale. This also applies to the six galaxies for which high-resolution mid-IR data are available. In the case of NGC 5194, we find that the non-thermal radio spectral index is correlated with the RC/FIR ratio. Conclusions: The scatter of the point-by-point correlations does not increase significantly with spatial resolution. We thus conclude that we have not yet probed the physical scales at which the correlations break down. However, we observe local deviations from the correlations in regions with a high star formation rate, such as the spiral arms, where we observe a flat radio spectrum and a low RC/FIR ratio. In the intra-arm regions and in the peripheral regions of the disk, the RC/FIR is generally higher and it is characterized by a steepening of the radio spectrum.

astro-ph↗

arXiv2006

A multi-resolution analysis of the radio-FIR correlation in the Large Magellanic Cloud

We investigate the local correlation betwen the 1.4 GHz radio continuum and 60 micron far-infrared (FIR) emission within the Large Magellanic Cloud (LMC) on spatial scales between 0.05 and 1.5 kpc. On scales below ~1 kpc, the radio-FIR correlation is clearly better than the correlation of the cold gas tracers with either the radio or the FIR emission. For the LMC as a whole, there is a tight correlation between the radio and FIR emission on spatial scales above ~50 pc. By decomposing the radio emission into thermal and non-thermal components, however, we show that the scale on which the radio-FIR correlation breaks down is inversely proportional to the thermal fraction of the radio emission: regions that show a strong correlation to very small scales are the same regions where the thermal fraction of the radio emission is high. Contrary to previous studies of the local radio-FIR correlation in the LMC, we show that the slope of the relation between the radio and FIR emission is non-linear. In bright star-forming regions, the radio emission increases faster than linearly with respect to the FIR emission (power-law slope of ~1.2), whereas a flatter slope of ~0.6-0.9 applies more generally across the LMC. Our results are consistent with a scenario in which the UV photons and cosmic rays in the LMC have a common origin in massive star formation, but the cosmic rays are able to diffuse away from their production sites. Our results do not provide direct evidence for coupling between the magnetic field and the local gas density, but we note that synchrotron emission may not be a good tracer of the magnetic field if cosmic rays can readily escape the LMC.

astro-ph↗

arXiv2005

The Molecular Connection to the FIR-Radio Continuum Correlation in Galaxies

We have studied the relationships between the radio continuum (RC) and CO emission for a set of galaxies selected from the BIMA Survey of Nearby Galaxies. We find that the global CO-RC correlation is as tight as the global FIR-RC correlation for the 24 galaxies studied. Within 9 galaxies with ~6 arcsec CO and RC data available, the CO and RC emission is as tightly correlated as its global value; the radially averaged correlation is nearly linear, extends over four order of magnitude and holds down to the smallest linear resolution of the observations, which is ~100 pc. We define qco as the log of the ratio of the CO to RC flux as a way to characterize the CO-RC correlation. Combining 6 arcsec pixel-by-pixel comparisons across all sources yields an average small-scale correlation of qco = 1.1 +/- 0.28; that is, the spatially resolved correlation has a dispersion that is less than a factor of 2. There are however systematic variations in the CO/RC ratio; the strongest organized structures in qco tend to be found along spiral arms and on size scales much larger than the resolution of the observations. We do not measure any systematic trend in CO/RC ratio as a function of radius in galaxies. The constancy of the CO/RC ratio stands in contrast to the previously measured decrease in the FIR/RC ratio as a function of radius in galaxies. We suggest that the excellent correlation between the CO, RC and FIR emission in galaxies is a consequence of regulation by hydrostatic pressure; this model links all three emissions without invoking an explicit dependence on a star formation scenario.

astro-ph↗

arXiv2001

The Nature of the Faint Radio Population

We present the results obtained so far from the optical follow-up of the ATESP sub-mJy radio survey. The ATESP results are then compared with the ones obtained from other deep radio samples and we show how the existing discrepancies about the nature of the faint radio population can be explained in terms of selection effects.

astro-ph↗

arXiv2001

HIPASS Detection of an Intergalactic Gas Cloud in the NGC 2442 Group

We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of a gas cloud associated with the asymmetric spiral galaxy NGC 2442. This object, designated HIPASS J0731-69, contains ~10^9 M_sun of HI, or nearly one-third as much atomic gas as NGC 2442 itself. No optical counterpart to any part of HIPASS J0731-69 has yet been identified, consistent with the gas being diffuse, and with its stream-like kinematics. If the gas in HIPASS J0731-69 was once part of NGC 2442, then it was most likely a fairly recent tidal encounter with a moderately massive companion which tore it loose, although the possibility of ram-pressure stripping cannot be ruled out. This discovery highlights the potential of the HIPASS data for yielding new clues to the nature of some of the best-known galaxies in the local universe.

astro-ph↗