arXiv subjects

J. D. Collier

Publications and source records attributed to J. D. Collier.

At least 37 records · Page 2Linked to original sources

arXiv2021

Australian Square Kilometre Array Pathfinder: I. System Description

In this paper we describe the system design and capabilities of the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope at the conclusion of its construction project and commencement of science operations. ASKAP is one of the first radio telescopes to deploy phased array feed (PAF) technology on a large scale, giving it an instantaneous field of view that covers 31 square degrees at 800 MHz. As a two-dimensional array of 36x12m antennas, with baselines ranging from 22m to 6km, ASKAP also has excellent snapshot imaging capability and 10 arcsecond resolution. This, combined with 288 MHz of instantaneous bandwidth and a unique third axis of rotation on each antenna, gives ASKAP the capability to create high dynamic range images of large sky areas very quickly. It is an excellent telescope for surveys between 700 MHz and 1800 MHz and is expected to facilitate great advances in our understanding of galaxy formation, cosmology and radio transients while opening new parameter space for discovery of the unknown.

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arXiv2021

Remnant Radio Galaxies Discovered in a Multi-frequency Survey

The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a $8.31$\,deg$^2$ sub-region of the GAMA~23~field which comprises of surveys covering the frequency range 0.1--9\,GHz. We present a sample of 104 radio galaxies compiled from observations conducted by the Murchison Wide-field Array (216\,MHz), the Australia Square Kilometer Array Pathfinder (887\,MHz), and the Australia Telescope Compact Array (5.5\,GHz). We adopt an `absent radio core' criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5\,GHz; at this frequency the emission is short-lived, implying a recent jet switch-off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep spectrum ($α<-1.2$) and a diffuse, low surface-brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our sample of radio galaxies, our observations constrain the remnant fraction to $4\%\lesssim f_{\mathrm{rem}} \lesssim 10\%$; the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5--10\,Myr at 5.5\,GHz after the jets switch off -- radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.

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arXiv2021

The infrared-radio correlation of star-forming galaxies is strongly M$_{\star}$-dependent but nearly redshift-invariant since z$\sim$4

Several works in the past decade have used the ratio between total (rest 8-1000$μ$m) infrared and radio (rest 1.4~GHz) luminosity in star-forming galaxies (q$_{IR}$), often referred to as the "infrared-radio correlation" (IRRC), to calibrate radio emission as a star formation rate (SFR) indicator. Previous studies constrained the evolution of q$_{IR}$ with redshift, finding a mild but significant decline, that is yet to be understood. For the first time, we calibrate q$_{IR}$ as a function of \textit{both} stellar mass (M$_{\star}$) and redshift, starting from an M$_{\star}$-selected sample of $>$400,000 star-forming galaxies in the COSMOS field, identified via (NUV-r)/(r-J) colours, at redshifts 0.1$<$z$<$4.5. Within each (M$_{\star}$,z) bin, we stack the deepest available infrared/sub-mm and radio images. We fit the stacked IR spectral energy distributions with typical star-forming galaxy and IR-AGN templates, and carefully remove radio AGN candidates via a recursive approach. We find that the IRRC evolves primarily with M$_{\star}$, with more massive galaxies displaying systematically lower q$_{IR}$. A secondary, weaker dependence on redshift is also observed. The best-fit analytical expression is the following: q$_{IR}$(M$_{\star}$,z)=(2.646$\pm$0.024)$\times$(1+z)$^{(-0.023\pm0.008)}$-(0.148$\pm$0.013)$\times$($\log~M_{\star}$/M$_{\odot}$-10). The lower IR/radio ratios seen in more massive galaxies are well described by their higher observed SFR surface densities. Our findings highlight that using radio-synchrotron emission as a proxy for SFR requires novel M$_{\star}$-dependent recipes, that will enable us to convert detections from future ultra deep radio surveys into accurate SFR measurements down to low-SFR, low-M$_{\star}$ galaxies.

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arXiv2021

Evolutionary Map of the Universe (EMU):Compact radio sources in the SCORPIO field towards the Galactic plane

We present observations of a region of the Galactic plane taken during the Early Science Program of the Australian Square Kilometre Array Pathfinder (ASKAP). In this context, we observed the SCORPIO field at 912 MHz with an uncompleted array consisting of 15 commissioned antennas. The resulting map covers a square region of ~40 deg^2, centred on (l, b)=(343.5°, 0.75°), with a synthesized beam of 24"x21" and a background rms noise of 150-200 μJy/beam, increasing to 500-600 μJy/beam close to the Galactic plane. A total of 3963 radio sources were detected and characterized in the field using the CAESAR source finder. We obtained differential source counts in agreement with previously published data after correction for source extraction and characterization uncertainties, estimated from simulated data. The ASKAP positional and flux density scale accuracy were also investigated through comparison with previous surveys (MGPS, NVSS) and additional observations of the SCORPIO field, carried out with ATCA at 2.1 GHz and 10" spatial resolution. These allowed us to obtain a measurement of the spectral index for a subset of the catalogued sources and an estimated fraction of (at least) 8% of resolved sources in the reported catalogue. We cross-matched our catalogued sources with different astronomical databases to search for possible counterparts, finding ~150 associations to known Galactic objects. Finally, we explored a multiparametric approach for classifying previously unreported Galactic sources based on their radio-infrared colors.

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arXiv2020

Radio observations of the merging galaxy cluster system Abell 3391-Abell 3395

The pre-merging system of galaxy clusters Abell 3391-Abell 3395 located at a mean redshift of 0.053 has been observed at 1 GHz in an ASKAP/EMU Early Science observation as well as in X-rays with eROSITA. The projected separation of the X-ray peaks of the two clusters is $\sim$50$'$ or $\sim$ 3.1 Mpc. Here we present an inventory of interesting radio sources in this field around this cluster merger. While the eROSITA observations provide clear indications of a bridge of thermal gas between the clusters, neither ASKAP nor MWA observations show any diffuse radio emission coinciding with the X-ray bridge. We derive an upper limit on the radio emissivity in the bridge region of $\langle J \rangle_{1\,{\rm GHz}}< 1.2 \times 10^{-44} {\rm W}\, {\rm Hz}^{-1} {\rm m}^{-3}$. A non-detection of diffuse radio emission in the X-ray bridge between these two clusters has implications for particle-acceleration mechanisms in cosmological large-scale structure. We also report extended or otherwise noteworthy radio sources in the 30 deg$^2$ field around Abell 3391-Abell 3395. We identified 20 Giant Radio Galaxies, plus 7 candidates, with linear projected sizes greater than 1 Mpc. The sky density of field radio galaxies with largest linear sizes of $>0.7$ Mpc is $\approx 1.7$ deg$^{-2}$, three times higher than previously reported. We find no evidence for a cosmological evolution of the population of Giant Radio Galaxies. Moreover, we find seven candidates for cluster radio relics and radio halos.

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arXiv2020

The Abell 3391/95 galaxy cluster system: A 15 Mpc intergalactic medium emission filament, a warm gas bridge, infalling matter clumps, and (re-) accelerated plasma discovered by combining SRG/eROSITA data with ASKAP/EMU and DECam data

We used dedicated SRG/eROSITA X-ray, ASKAP/EMU radio, and DECam optical observations of a 15 sq.deg region around the interacting galaxy cluster system A3391/95 to study the warm-hot gas in cluster outskirts and filaments, the surrounding large-scale structure and its formation process. We relate the observations to expectations from cosmological hydrodynamic simulations from the Magneticum suite. We trace the irregular morphology of warm-hot gas of the main clusters from their centers out to well beyond their characteristic radii, $r_{200}$. Between the two main cluster systems, we observe an emission bridge; thanks to eROSITA's unique soft response and large field of view, we discover tantalizing hints for warm gas. Several matter clumps physically surrounding the system are detected. For the "Northern Clump," we provide evidence that it is falling towards A3391 from the hot gas morphology and radio lobe structure of its central AGN. Many of the extended sources in the field detected by eROSITA are known clusters or new clusters in the background, including a known SZ cluster at redshift z=1. We discover an emission filament north of the virial radius, $r_{100}$, of A3391 connecting to the Northern Clump and extending south of A3395 towards another galaxy cluster. The total projected length of this continuous warm-hot emission filament is 15 Mpc, running almost 4 degrees across the entire eROSITA observation. The DECam galaxy density map shows galaxy overdensities in the same regions. The new datasets provide impressive confirmation of the theoretically expected structure formation processes on the individual system level, including the surrounding warm-hot intergalactic medium distribution compared to the Magneticum simulation. Our spatially resolved findings show that baryons indeed reside in large-scale warm-hot gas filaments with a clumpy structure.

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arXiv2020

MIGHTEE: Are giant radio galaxies more common than we thought?

We report the discovery of two new giant radio galaxies (GRGs) using the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. Both GRGs were found within a 1 deg^2 region inside the COSMOS field. They have redshifts of z=0.1656 and z=0.3363 and physical sizes of 2.4Mpc and 2.0Mpc, respectively. Only the cores of these GRGs were clearly visible in previous high resolution VLA observations, since the diffuse emission of the lobes was resolved out. However, the excellent sensitivity and uv coverage of the new MeerKAT telescope allowed this diffuse emission to be detected. The GRGs occupy a unpopulated region of radio power - size parameter space. Based on a recent estimate of the GRG number density, the probability of finding two or more GRGs with such large sizes at z<0.4 in a ~1deg^2 field is only 2.7x10^-6, assuming Poisson statistics. This supports the hypothesis that the prevalence of GRGs has been significantly underestimated in the past due to limited sensitivity to low surface brightness emission. The two GRGs presented here may be the first of a new population to be revealed through surveys like MIGHTEE which provide exquisite sensitivity to diffuse, extended emission.

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arXiv2020

The Rapid ASKAP Continuum Survey I: Design and First Results

The Rapid ASKAP Continuum Survey (RACS) is the first large-area survey to be conducted with the full 36-antenna Australian Square Kilometre Array Pathfinder (ASKAP) telescope. RACS will provide a shallow model of the ASKAP sky that will aid the calibration of future deep ASKAP surveys. RACS will cover the whole sky visible from the ASKAP site in Western Australia, and will cover the full ASKAP band of $700-1800$ MHz. The RACS images are generally deeper than the existing NRAO VLA Sky Survey (NVSS) and Sydney University Molonglo Sky Survey (SUMSS) radio surveys and have better spatial resolution. All RACS survey products will be public, including radio images (with $\sim 15$ arcsecond resolution) and catalogues of about three million source components with spectral index and polarisation information. In this paper, we present a description of the RACS survey and the first data release of 903 images covering the sky south of declination $+41^\circ$ made over a 288 MHz band centred at 887.5 MHz.

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arXiv2020

MIGHTEE-HI: The HI emission project of the MeerKAT MIGHTEE survey

We present the HI emission project within the MIGHTEE survey, currently being carried out with the newly commissioned MeerKAT radio telescope. This is one of the first deep, blind, medium-wide interferometric surveys for neutral hydrogen (HI) ever undertaken, extending our knowledge of HI emission to z=0.6. The science goals of this medium-deep, medium-wide survey are extensive, including the evolution of the neutral gas content of galaxies over the past 5 billion years. Simulations predict nearly 3000 galaxies over 0<z<0.4 will be detected directly in HI, with statistical detections extending to z=0.6. The survey allows us to explore HI as a function of galaxy environment, with massive groups and galaxy clusters within the survey volume. Additionally, the area is large enough to contain as many as 50 local galaxies with HI mass $<10^8$ Msun, which allows us to study the low-mass galaxy population. The 20 deg$^2$ main survey area is centred on fields with exceptional multi-wavelength ancillary data, with photometry ranging from optical through far-infrared wavelengths, supplemented with multiple spectroscopic campaigns. We describe here the survey design and the key science goals. We also show first results from the Early Science observations, including kinematic modelling of individual sources, along with the redshift, HI, and stellar mass ranges of the sample to date.

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arXiv2020

The relation between the diffuse X-ray luminosity and the radio power of the central AGN in galaxy groups

Our understanding of how AGN feedback operates in galaxy clusters has improved in recent years owing to large efforts in multi-wavelength observations and hydrodynamical simulations. However, it is much less clear how feedback operates in galaxy groups, which have shallower gravitational potentials. In this work, using very deep VLA and new MeerKAT observations from the MIGHTEE survey, we compiled a sample of 247 X-ray selected galaxy groups detected in the COSMOS field. We have studied the relation between the X-ray emission of the intra-group medium and the 1.4 GHz radio emission of the central radio galaxy. For comparison, we have also built a control sample of 142 galaxy clusters using ROSAT and NVSS data. We find that clusters and groups follow the same correlation between X-ray and radio emission. Large radio galaxies hosted in the centres of groups and merging clusters increase the scatter of the distribution. Using statistical tests and Monte-Carlo simulations, we show that the correlation is not dominated by biases or selection effects. We also find that galaxy groups are more likely than clusters to host large radio galaxies, perhaps owing to the lower ambient gas density or a more efficient accretion mode. In these groups, radiative cooling of the ICM could be less suppressed by AGN heating. We conclude that the feedback processes that operate in galaxy clusters are also effective in groups.

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arXiv2020

PKS 2250$-$351: A Giant Radio Galaxy in Abell 3936

We present a detailed analysis of the radio galaxy PKS 2250-351, a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and infra-red data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS 2250-351 lies at z=0.2115 in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, `red and dead' elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-infrared emission. Assuming it lies on the local M-sigma relation it has an Eddington accretion rate of lambda_EDD~0.014. We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission (~300 Myr) the accretion has switched from an inefficient advection dominated mode to a thin-disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS 2250-351 lying to the west of the densest concentration of galaxies in Abell 3936.

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arXiv2019

The ASKAP-EMU Early Science Project:Radio Continuum Survey of the Small Magellanic Cloud

We present two new radio continuum images from the Australian Square Kilometre Array Pathfinder (ASKAP) survey in the direction of the Small Magellanic Cloud (SMC). These images are part of the Evolutionary Map of the Universe (EMU) Early Science Project (ESP) survey of the Small and Large Magellanic Clouds. The two new source lists produced from these images contain radio continuum sources observed at 960 MHz (4489 sources) and 1320 MHz (5954 sources) with a bandwidth of 192 MHz and beam sizes of 30.0"x30.0" and 16.3"x15.1", respectively. The median Root Mean Squared (RMS) noise values are 186$μ$Jy beam$^{-1}$ (960 MHz) and 165$μ$Jy beam$^{-1}$ (1320 MHz). To create point source catalogues, we use these two source lists, together with the previously published Molonglo Observatory Synthesis Telescope (MOST) and the Australia Telescope Compact Array (ATCA) point source catalogues to estimate spectral indices for the whole population of radio point sources found in the survey region. Combining our ASKAP catalogues with these radio continuum surveys, we found 7736 point-like sources in common over an area of 30 deg$^2$. In addition, we report the detection of two new, low surface brightness supernova remnant candidates in the SMC. The high sensitivity of the new ASKAP ESP survey also enabled us to detect the bright end of the SMC planetary nebula sample, with 22 out of 102 optically known planetary nebulae showing point-like radio continuum emission. Lastly, we present several morphologically interesting background radio galaxies.

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arXiv2019

ASKAP Commissioning Observations of the GAMA 23 Field

We have observed the G23 field of the Galaxy And Mass Assembly (GAMA) survey using the Australian Square Kilometre Array Pathfinder (ASKAP) in its commissioning phase, to validate the performance of the telescope and to characterize the detected galaxy populations. This observation covers $\sim$48 deg$^2$ with synthesized beam of 32.7$^{\prime\prime}$ by 17.8$^{\prime\prime}$ at 936 MHz, and $\sim$39 deg$^2$ with synthesized beam of 15.8$^{\prime\prime}$ by 12.0$^{\prime\prime}$ at 1320 MHz. At both frequencies, the r.m.s. (root-mean-square) noise is $\sim$0.1 mJy/beam. We combine these radio observations with the GAMA galaxy data, which includes spectroscopy of galaxies that are i-band selected with a magnitude limit of 19.2. Wide-field Infrared Survey Explorer (WISE) infrared (IR) photometry is used to determine which galaxies host an active galactic nucleus (AGN). In properties including source counts, mass distributions, and IR vs. radio luminosity relation, the ASKAP detected radio sources behave as expected. Radio galaxies have higher stellar mass and luminosity in IR, optical and UV than other galaxies. We apply optical and IR AGN diagnostics and find that they disagree for $\sim$30% of the galaxies in our sample. We suggest possible causes for the disagreement. Some cases can be explained by optical extinction of the AGN, but for more than half of the cases we do not find a clear explanation. Radio sources are more likely ($\sim$6%) to have an AGN than radio quiet galaxies ($\sim$1%), but the majority of AGN are not detected in radio at this sensitivity.

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arXiv2019

The Performance and Calibration of the CRAFT Fly's Eye Fast Radio Burst Survey

Since January 2017, the Commensal Real-time ASKAP Fast Transients survey (CRAFT) has been utilising commissioning antennas of the Australian SKA Pathfinder (ASKAP) to survey for fast radio bursts (FRBs) in fly's eye mode. This is the first extensive astronomical survey using phased array feeds (PAFs), and a total of 20 FRBs have been reported. Here we present a calculation of the sensitivity and total exposure of this survey, using the pulsars B1641-45 (J1644-4559) and B0833-45 (J0835-4510, i.e.\ Vela) as calibrators. The design of the survey allows us to benchmark effects due to PAF beamshape, antenna-dependent system noise, radio-frequency interference, and fluctuations during commissioning on timescales from one hour to a year. Observation time, solid-angle, and search efficiency are calculated as a function of FRB fluence threshold. Using this metric, effective survey exposures and sensitivities are calculated as a function of the source counts distribution. The implied FRB rate is significantly lower than the $37$\,sky$^{-1}$\,day$^{-1}$ calculated using nominal exposures and sensitivities for this same sample by \citet{craft_nature}. At the Euclidean power-law index of $-1.5$, the rate is $10.7_{-1.8}^{+2.7}\,{\rm (sys)} \, \pm \, 3\,{\rm (stat)}$\,sky$^{-1}$\,day$^{-1}$ above a threshold of $57\pm6\,{\rm (sys)}$\,Jy\,ms, while for the best-fit index for this sample of $-2.1$, it is $16.6_{-1.5}^{+1.9} \,{\rm (sys)}\, \pm 4.7\,{\rm (stat)}$\,sky$^{-1}$\,day$^{-1}$ above a threshold of $41.6\pm1.5\,{\rm (sys)}$\,Jy\,ms. This strongly suggests that these calculations be performed for other FRB-hunting experiments, allowing meaningful comparisons to be made between them.

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arXiv2019

WALLABY Early Science - II. The NGC 7232 galaxy group

We report on neutral hydrogen (HI) observations of the NGC 7232 group with the Australian Square Kilometre Array Pathfinder (ASKAP). These observations were conducted as part of the Wide-field ASKAP L-Band Legacy All-sky Blind surveY (WALLABY) Early Science program with an array of 12 ASKAP antennas equipped with Phased Array Feeds, which were used to form 36 beams to map a field of view of 30 square degrees. Analyzing a subregion of the central beams, we detect 17 HI sources. Eleven of these detections are identified as galaxies and have stellar counterparts, of which five are newly resolved HI galaxy sources. The other six detections appear to be tidal debris in the form of HI clouds that are associated with the central triplet, NGC 7232/3, comprising the spiral galaxies NGC 7232, NGC7232B and NGC7233. One of these HI clouds has a mass of M_HI ~ 3 x 10^8 M_sol and could be the progenitor of a long-lived tidal dwarf galaxy. The remaining HI clouds are likely transient tidal knots that are possibly part of a diffuse tidal bridge between NGC 7232/3 and another group member, the lenticular galaxy IC 5181.

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arXiv2018

Cold gas outflows from the Small Magellanic Cloud traced with ASKAP

Feedback from massive stars plays a critical role in the evolution of the Universe by driving powerful outflows from galaxies that enrich the intergalactic medium and regulate star formation. An important source of outflows may be the most numerous galaxies in the Universe: dwarf galaxies. With small gravitational potential wells, these galaxies easily lose their star-forming material in the presence of intense stellar feedback. Here, we show that the nearby dwarf galaxy, the Small Magellanic Cloud (SMC), has atomic hydrogen outflows extending at least 2 kiloparsecs (kpc) from the star-forming bar of the galaxy. The outflows are cold, $T<400~{\rm K}$, and may have formed during a period of active star formation $25 - 60$ million years (Myr) ago. The total mass of atomic gas in the outflow is $\sim 10^7$ solar masses, ${\rm M_{\odot}}$, or $\sim 3$% of the total atomic gas of the galaxy. The inferred mass flux in atomic gas alone, $\dot{M}_{HI}\sim 0.2 - 1.0~{\rm M_{\odot}~yr^{-1}}$, is up to an order of magnitude greater than the star formation rate. We suggest that most of the observed outflow will be stripped from the SMC through its interaction with its companion, the Large Magellanic Cloud (LMC), and the Milky Way, feeding the Magellanic Stream of hydrogen encircling the Milky Way.

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arXiv2018

A Morphological Study of the Supernova Remnant RX J0852.0-4622 (Vela Jr.)

We conduct a multi-wavelength morphological study of the Galactic supernova remnant RXJ0852.0-4622 (also known as Vela Jr., Vela Z and G266.2-1.2). RX J0852.0-4622 is coincident with the edge of the larger Vela supernova remnant causing confusion in the attribution of some filamentary structures to either RX J0852.0-4622 or its larger sibling. We find that the RX J0852.0-4622 radio continuum emission can be characterised by a 2-dimensional shell with a radius of 0.90+/-0.01deg (or 11.8+/-0.6pc at an assumed distance of 750pc) centred at (l,b)=(133.08+/-0.01 deg,-46.34+/-0.01deg) (or RA=8h52m19.2s, Dec=-46deg20'24.0'', J2000), consistent with X-ray and gamma-ray emission. Although [OIII] emission features are generally associated with the Vela SNR, one particular [OIII] emission feature, which we denote as "the Vela Claw", morphologically matches a molecular clump that is thought to have been stripped by the stellar progenitor of the RX J0852.0-4622 SNR. We argue that the Vela Claw feature is possibly associated with RX J0852.0-4622. Towards the north-western edge of RX J0852.0-4622, we find a flattening of the radio spectral index towards another molecular clump also thought to be associated with RX J0852.0-4622. It is currently unclear whether this feature and the Vela Claw result from interactions between the RX J0852.0-4622 shock and the ISM.

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arXiv2018

A pilot survey for transients and variables with the Australian Square Kilometre Array Pathfinder

We present a pilot search for variable and transient sources at 1.4 GHz with the Australian Square Kilometre Array Pathfinder (ASKAP). The search was performed in a 30 deg$^{2}$ area centred on the NGC 7232 galaxy group over 8 epochs and observed with a near-daily cadence. The search yielded nine potential variable sources, rejecting the null hypothesis that the flux densities of these sources do not change with 99.9% confidence. These nine sources displayed flux density variations with modulation indices m $\geq 0.1$ above our flux density limit of 1.5 mJy. They are identified to be compact AGN/quasars or galaxies hosting an AGN, whose variability is consistent with refractive interstellar scintillation. We also detect a highly variable source with modulation index m $ > 0.5$ over a time interval of a decade between the Sydney University Molonglo Sky Survey (SUMSS) and our latest ASKAP observations. We find the source to be consistent with the properties of long-term variability of a quasar. No transients were detected on timescales of days and we place an upper limit $ρ< 0.01$ deg$^{2}$ with 95% confidence for non-detections on near-daily timescales. The future VAST-Wide survey with 36-ASKAP dishes will probe the transient phase space with a similar cadence to our pilot survey, but better sensitivity, and will detect and monitor rarer brighter events.

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