arXiv subjects

G. Heald

Publications and source records attributed to G. Heald.

At least 19 recordsLinked to original sources

arXiv2026

Investigating the magnetic field in the inter-cluster filament between Abell 3667 and Abell 3651 with POSSUM

[Abridged abstract] The objective of this study is to measure the magnetic field within the prominent inter-cluster filament recently detected in X-rays by the extended ROentgen Survey with an Imaging Telescope Array (eROSITA). This filament spans over 13 Mpc projected on the sky, connecting the galaxy clusters Abell 3667 and Abell 3651. We employed the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) Rotation Measure (RM) grid to isolate the RM dispersion and median value of background polarised sources induced by the filament's magnetised plasma, and to infer the strength of this magnetic field. The filament region is sampled by 54 background polarised sources. After subtracting the foreground Galactic RM, we detected a marginal residual RM dispersion in the filament region of $6.9\pm3.6$ rad/m$^{2}$, together with a coherent residual RM signal with median $6.3\pm1.3$ rad/m$^{2}$. Assuming simplified single-scale magnetic-field models and adopting informed priors on the thermal electron density distribution derived from the X-ray analysis, we constrained the magnetic field strength to the range 0.1-3.5 $\mu$G within 95$\%$ confidence, with preferred values around 0.2-0.3 $\mu$G depending on the assumed magnetic-field coherence scale. However, we also found that the Galactic foreground RM in this region is highly structured on angular scales comparable to the extent of the filament itself, representing a major source of uncertainty for the RM analysis. Our results provide the first magnetic field constraints based on Faraday rotation measurements in an individual X-ray-detected inter-cluster filament, with field strengths consistent with theoretical expectations for gas in bridges and cluster outskirts. Our analysis also highlights the critical importance of accurately modelling Galactic RM foregrounds for future studies of extra-galactic magnetism with POSSUM and the SKA.

astro-ph.CO↗

arXiv2026

Helical radio jets as probes of magnetised cluster environments: Periodic Faraday Rotation Revealed in the Corkscrew Galaxy by POSSUM

Jets from active galactic nuclei (AGN) that exhibit regular plane-of-sky oscillations may provide a probe of magnetised plasma within the jets and their local intracluster medium (ICM). If such morphologies trace three-dimensional helices, path length differences between near and far sides of the flow might imprint quasi-periodic signatures in Faraday rotation. We present ASKAP Band 2 (1296-1440 MHz) polarimetric observations of a helical-tailed radio galaxy (the "Corkscrew Galaxy") from early science data from the POlarisation Sky Survey of the Universe's Magnetism (POSSUM), and test whether its quasi-periodic morphology produces signatures in Faraday rotation. We detect significant RM oscillations with a spatial period of (342 +/- 101) arcsec, consistent within the Rayleigh resolution limit with the jet's lateral deviations ((290 +/- 72) arcsec). Cross-correlation analysis reveals systematic variation along the jet: the eastern section shows alignment between jet deviation and RM, consistent with a jet-associated or sheath-like Faraday screen, while the western section exhibits a phase shift, indicating a transition in the dominant Faraday-rotating medium and possible contribution from the local ICM. These results demonstrate that quasi-periodic RM signatures can disentangle the dominant Faraday-rotating medium in AGN jets, and show that in some embedded radio galaxies the dominant RM contribution arises near the source rather than the cluster foreground. We also outline reliability criteria required to avoid false positive detections and inadequate sampling. Helical jet structures therefore show strong promise as probes of magnetised cluster environments, a capability that will expand in the SKA era.

astro-ph.GA↗

arXiv2026

Periodic Radio and X-ray Emission from an Accreting White Dwarf Binary

Long period radio transients (LPTs) are coherent bursts of polarised radio emission that repeat periodically on timescales of minutes to hours. Little is known about the physical origins of these systems. Astronomers have proposed magnetars that rotate slowly and white dwarfs that rapidly orbit with a companion star as potential explanations. While several recent examples appear to support the latter hypothesis, the mechanism generating these bright radio pulses remains poorly understood. Here we report our discovery and classification of the LPT ASKAP J174508.9-505149 as an accreting white dwarf binary. This object has a ~1.3h spectroscopic orbital period and exhibits orbitally-modulated X-ray emission and radio bursts. These elliptically polarised radio bursts drift in emission frequency, potentially due to a longer beat period, and turn off for several hours at a time. Some long period radio transients have been associated with non-interacting white dwarf binaries. We have spectroscopically confirmed this system as an accreting cataclysmic variable, identified through characteristic optical emission lines and an ongoing X-ray outburst. Our results strengthen the link between at least some long period radio transients and white dwarf binaries.

astro-ph.HE↗

arXiv2026

Magnetised CGM Gas at z~1 revealed by SPICE-RACS

Magnetic fields are expected to permeate the circumgalactic medium (CGM) of galaxies, yet direct constraints at high redshift remain limited by the lack of high-quality Faraday rotation measure (RM) data. Using the RMs from SPICE-RACS DR2 combined with the DESI DR1 quasar catalogue, we compile the largest sample to date of 2483 quasar sightlines with associated RMs, including 612 with intervening Mg II absorbers tracing foreground galaxies and 1871 control sightlines without Mg II absorbers. After subtracting the Galactic RM contribution and restricting the analysis to sightlines with low Milky Way HI column density and H$\alpha$ intensity, we obtain a foreground-cleaned sample of 757 quasars (191 Mg II / 566 control) spanning redshifts $0.13<z<3.45$. In this foreground-cleaned sample, Mg II sightlines exhibit a $4.5\sigma$ excess in the residual RM dispersion of $4.13 \pm 0.91~\mathrm{rad\,m^{-2}}$ relative to the control sample, at a median absorber redshift of $z\sim1.14$. This implies model-dependent CGM magnetic field strengths of $\sim0.4 - 0.8\, \mu$G over projected radii of $20 - 150$ kpc. This indicates that substantial CGM magnetisation was already established by $z\sim1$, enabling new constraints on the growth and amplification of magnetic fields in galaxy halos over cosmic time.

astro-ph.GA↗

arXiv2025

GLEAM-300: The GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey at 300 MHz

In this paper we present a wide-field radio survey at 300 MHz covering the sky from $-90^\circ \leq \delta_\text{J2000} \lesssim +40^\circ$ using the Murchison Widefield Array (MWA). This 300-MHz survey follows the Galactic and Extragalactic All-sky MWA (GLEAM) survey, and provides an additional comparatively high-frequency data point to existing multi-frequency (72-231 MHz) data. With this data release we provide mosaic images and a catalogue of compact source components. We use two-minute snapshot observations covering 2015-2016, combining overlapping two-minute snapshot images to provide full-sensitivity mosaic images with a median root-mean-square noise of $9.1_{-2.8}^{+5.5}$ mJy beam$^{-1}$ and median angular resolution of $128.8^{\prime\prime} \times 112.5^{\prime\prime}$, with some position-dependent variation. We find a total of 338 080 unique Gaussian components across the mosaic images. The survey is the first at 300 MHz from the MWA covering the whole Southern Hemisphere. It provides a unique spectral data point that complements the existing GLEAM survey and the ongoing GLEAM-eXtended survey, and points toward results from the upcoming SKA-Low surveys.

astro-ph.GA↗

arXiv2024

On the nature of LOFAR RMs and new constraints on magnetic fields in cosmic filaments and on magnetogenesis scenarios

The measurement of magnetic fields in cosmic web filaments can be used to reveal the magnetogenesis of the Universe. In previous work, we produced first estimates of the field strength and its redshift evolution using the Faraday Rotation Measure (RM) catalogue of extragalactic background sources at low frequency obtained with LOFAR observations. Here we refine our analysis by selecting sources with low Galactic RM, which reduces its residual contamination. We also conduct a comprehensive analysis of the different contributions to the extragalactic RMs along the line of sight, and confirm that they are dominated by the cosmic filaments component, with only 21 percent originating in galaxy clusters and the circumgalactic medium (CGM) of galaxies. We find a possible hint of a shock at the virial radius of massive galaxies. We also find that the fractional polarization of background sources might be a valuable CGM tracer. The newly selected RMs have a steeper evolution with redshift than previously found. The field strength in filaments ($B_f$) and its evolution are estimated assuming $B_f$ evolves as a power-law $B_f=B_{f,0}\,(1+z)^\alpha$. Our analysis finds an average strength at $z=0$ of $B_{f,0} =11$--15~nG, with an error of 4 nG, and a slope $\alpha=2.3$--$2.6 \pm 0.5$, which is steeper than what we previously found. The comoving field has a slope of $\beta=$ [0.3, 0.6$]\pm 0.5$ that is consistent with being invariant with redshift. Primordial magnetogenesis scenarios are favoured by our data, together with a sub-dominant astrophysical-origin RM component increasing with redshift.

astro-ph.CO↗

arXiv2024

The twisted jets and magnetic fields of the extended radio galaxy 4C 70.19

The appearance of the jets and lobes of some radio galaxies makes it difficult to assign them to a known class of objects. This is often due to the activity of the central engine and/or interactions with the environment, as well as projection effects. We analyse the radio data for an apparently asymmetric radio source 4C70.19, which is associated with the giant elliptical galaxy NGC6048. The source shows distorted radio jets and lobes, one of which bends by 180 degrees. The aim of our study is to explain the nature of the observed distortions. We used LOFAR, Effelsberg, and VLA radio data in a wide range of frequencies. At high frequencies, we also used radio polarimetry to study the properties of the magnetic fields. Additionally, we made use of optical, infra-red, and X-ray data. Polarisation data suggest shearing of the magnetic fields at points where the jets bend. The low-frequency LOFAR map at 145 MHz, as well as the sensitive single-dish Effelsberg map at 8.35 GHz, reveal previously undetected diffuse emission around the source. The rotation measure (RM) derived from the polarimetric data allowed us to estimate the density of the medium surrounding the source, which agrees with typical densities of the intergalactic medium or the outer parts of insterstellar halos. We propose that the southern jet is bent in the same manner as the northern one, but that it is inclined to the sky plane. Both these bends are likely caused by the orbital motion within the galaxy group, as well as interactions with the intergalactic medium. Our analyses suggest that, despite its complex morphology, 4C70.19 seems to be intrinsically symmetric with a physical extent of up to 600 kpc, and that the diffuse emission detected in our high-sensitivity maps is related to radio plumes that are expanding behind the source.

astro-ph.GA↗

arXiv2024

Magnetised HI superbubbles in the Small Magellanic Cloud revealed by the POSSUM pilot survey

Neutral hydrogen (HI) bubbles and shells are common in the interstellar medium (ISM). Studying their properties provides insight into the characteristics of the local ISM as well as the galaxy in which the bubbles reside. We report the detection of magnetic fields associated with superbubbles in the nearby irregular galaxy, the Small Magellanic Cloud (SMC). Using the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) pilot survey, we obtain a high-density grid ($\approx 25 \,\rm sources\,deg^{-2}$) of Faraday rotation measure (RM) from polarized sources behind the SMC. This provides a sufficiently large number of RM measurements to study the magnetic properties of three of the largest HI shells previously identified in the SMC. The RM profiles as a function of distance from the shell centre show characteristic patterns at angular scales comparable to the shell size. We demonstrate that this can be explained by magneto-hydrodynamic simulation models of bubbles expanding in magnetised environments. From the observations, we estimate the line-of-sight magnetic field strength at the edges of the shells is enhanced by $\sim1\,\rm \mu G$ with respect to their centres. This is an order of magnitude larger than the field strength in the ambient medium ($\sim 0.1\,\rm \mu G$) estimated based on the expansion velocity of the shells. This paper highlights the power of densely mapped RM grids in studying the magnetic properties of galactic substructures beyond the Milky Way.

astro-ph.GA↗

arXiv2024

Ram-pressure stripped radio tails detected in the dynamically active environment of the Shapley Supercluster

We study the radio continuum emission of four galaxies experiencing ram-pressure stripping in four clusters of the Shapley supercluster at redshift z~0.05. Multi-band (235-1367 MHz) radio data, complemented by integral-field spectroscopy, allow us to detect and analyse in detail the non-thermal component both in the galaxy discs and the radio continuum tails. Three galaxies present radio continuum tails which are tens of kiloparsecs long. By deriving the radio spectral index in the inner and outer tails and comparing our findings with the distribution of the extraplanar ionised gas and the results of N-body/hydrodynamical simulations, we demonstrate that these tails are caused by the ram pressure which, together with the ionised gas, sweeps the magnetic field from the galaxy discs. We suggest that the radio continuum emission in these tails can be differently powered by (i) in situ star formation; (ii) relativistic electrons stripped from the disc; (iii) shock excitation or a combination of them. All the ram-pressure stripped galaxies are found in environments where cluster-cluster interactions occurred and/or are ongoing thus strongly supporting the thesis that cluster and group collisions and mergers may locally increase the ram pressure and trigger hydrodynamical interactions between the intracluster medium and the interstellar medium of galaxies.

astro-ph.GA↗

arXiv2024

Probing the Magnetised Gas Distribution in Galaxy Groups and the Cosmic Web with POSSUM Faraday Rotation Measures

We present initial results from the Polarisation Sky Survey of the Universe's Magnetism (POSSUM), analysing 22,817 Faraday Rotation Measures (RMs) with median uncertainties of 1.2 rad m^-2 across 1,520 square degrees to study magnetised gas associated with 55 nearby galaxy groups (z less than 0.025) with halo masses between 10^12.5 and 10^14.0 M_sun. We identify two distinct gas phases: the Intragroup Medium (IGrM) within 0-2 splashback radii and the Warm-Hot Intergalactic Medium (WHIM) extending from 2 to 7 splashback radii. These phases enhance the standard deviation of residual (i.e., Galactic foreground RM-subtracted) RMs by 6.9 +/- 1.8 rad m^-2 and 4.2 +/- 1.2 rad m^-2, respectively. Estimated magnetic field strengths are several microGauss within the IGrM and 0.1-1 microGauss in the WHIM. We estimate the plasma beta in both phases and show that magnetic pressure might be more dynamically important than in the ICM of more massive clusters or sparse cosmic web filaments. Our findings indicate that "missing baryons" in the WHIM likely extend beyond the gravitational radii of group-mass halos to Mpc scales, consistent with large-scale, outflow-driven "magnetised bubbles" seen in cosmological simulations. We demonstrate that RM grids are an effective method for detecting magnetised thermal gas at galaxy group interfaces and within the cosmic web. This approach complements X-ray and Sunyaev-Zel'dovich effect methods, and when combined with Fast Radio Burst Dispersion Measures, data from the full POSSUM survey, comprising approximately a million RMs, will allow direct magnetic field measurements to further our understanding of baryon circulation in these environments and the magnetised universe.

astro-ph.GA↗

arXiv2024

GaLactic and Extragalactic All-sky Murchison Widefield Array eXtended (GLEAM-X) survey II: Second Data Release

We present the second data release for the GaLactic and Extragalactic All-sky Murchison Widefield Array eXtended (GLEAM-X) survey. This data release is an area of 12,892-deg^2 around the South Galactic Pole region covering 20h 40m <= RA <= 6h 40m, -90deg <= Dec <= +30deg. Observations were taken in 2020 using the Phase-II configuration of the Murchison Widefield Array (MWA) and covering a frequency range of 72-231MHz with twenty frequency bands. We produce a wideband source-finding mosaic over 170-231MHz with a median root mean squared noise of 1.5 (+1.5/-0.5) mJy beam^(-1). We present a catalogue of 624,866 components, including 562,302 components which are spectrally fit. This catalogue is 98% complete at 50mJy, and a reliability of 98.7% at a 5sigma level, consistent with expectations for this survey. The catalogue is made available via Vizier and the PASA datastore and accompanying mosaics for this data release are made available via AAO Data Central and SkyView.

astro-ph.GA↗

arXiv2024

MHONGOOSE -- A MeerKAT Nearby Galaxy HI Survey

The MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) survey maps the distribution and kinematics of the neutral atomic hydrogen (HI) gas in and around 30 nearby star-forming spiral and dwarf galaxies to extremely low HI column densities. The HI column density sensitivity (3 sigma over 16 km/s) ranges from ~ 5 x 10^{17} cm^{-2} at 90'' resolution to ~4 x 10^{19} cm^{-2} at the highest resolution of 7''. The HI mass sensitivity (3 sigma over 50 km/s) is ~5.5 X 10^5 M_sun at a distance of 10 Mpc (the median distance of the sample galaxies). The velocity resolution of the data is 1.4 km/s. One of the main science goals of the survey is the detection of cold, accreting gas in the outskirts of the sample galaxies. The sample was selected to cover a range in HI masses, from 10^7 M_sun to almost 10^{11} M_sun, to optimally sample possible accretion scenarios and environments. The distance to the sample galaxies ranges from 3 to 23 Mpc. In this paper, we present the sample selection, survey design, and observation and reduction procedures. We compare the integrated HI fluxes based on the MeerKAT data with those derived from single-dish measurement and find good agreement, indicating that our MeerKAT observations are recovering all flux. We present HI moment maps of the entire sample based on the first ten percent of the survey data, and find that a comparison of the zeroth- and second-moment values shows a clear separation between the physical properties of the HI in areas with star formation and areas without, related to the formation of a cold neutral medium. Finally, we give an overview of the HI-detected companion and satellite galaxies in the 30 fields, five of which have not previously been catalogued. We find a clear relation between the number of companion galaxies and the mass of the main target galaxy.

astro-ph.GA↗

arXiv2023

RMTable2023 and PolSpectra2023: standards for reporting polarization and Faraday rotation measurements of radio sources

Faraday rotation measures (RMs) have been used for many studies of cosmic magnetism, and in most cases having more RMs is beneficial for those studies. This has lead to development of RM surveys that have produced large catalogs, as well as meta-catalogs collecting RMs from many different publications. However, it has been difficult to take full advantage of all these RMs as the individual catalogs have been published in many different places, and in many different formats. In addition, the polarization spectra used to determine these RMs are rarely published, limiting the ability to re-analyze data as new methods or additional observations become available. We propose a standard convention for RM catalogs, RMTable2023, and a standard for source-integrated polarized spectra of radio sources, PolSpectra2023. These standards are intended to maximize the value and utility of these data for researchers and to make them easier to access. To demonstrate the use of the RMTable2023 standard, we have produced a consolidated catalog of 55 819 RMs collected from 42 published catalogs.

astro-ph.IM↗

arXiv2023

The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2

A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2461 extragalactic high-precision RM values across 5720 deg$^{2}$ of sky (corresponding to a polarized source areal number density of $\sim$0.43 deg$^{-2}$). The linear polarization and RM properties were derived using RM synthesis from the Stokes $Q$ and $U$ channel images at an angular resolution of 20'' across a frequency range of 120 to 168 MHz with a channel bandwidth of 97.6 kHz. The fraction of total intensity sources ($>1$ mJy beam$^{-1}$) found to be polarized was $\sim$0.2%. The median detection threshold was 0.6 mJy beam$^{-1}$ ($8\sigma_{QU}$), with a median RM uncertainty of 0.06 rad m$^{-2}$ (although a systematic uncertainty of up to 0.3 rad m$^{-2}$ is possible, after the ionosphere RM correction). The median degree of polarization of the detected sources is 1.8%, with a range of 0.05% to 31%. Comparisons with cm-wavelength RMs indicate minimal amounts of Faraday complexity in the LoTSS detections, making them ideal sources for RM Grid studies. Host galaxy identifications were obtained for 88% of the sources, along with redshifts for 79% (both photometric and spectroscopic), with the median redshift being 0.6. The focus of the current catalogue was on reliability rather than completeness, and we expect future versions of the LoTSS RM Grid to have a higher areal number density. In addition, 25 pulsars were identified, mainly through their high degrees of linear polarization.

astro-ph.CO↗

arXiv2022

Collimation of the kiloparsec-scale radio jets in NGC 2663

We present the discovery of highly-collimated radio jets spanning a total of 355 kpc around the nearby elliptical galaxy NGC 2663, and the possible first detection of recollimation on kiloparsec scales. The small distance to the galaxy (~28.5 Mpc) allows us to resolve portions of the jets to examine their structure. We combine multiwavelength data: radio observations by the Murchison Widefield Array (MWA), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Australia Telescope Compact Array (ATCA), and X-ray data from Chandra, Swift and SRG/eROSITA. We present intensity, rotation measure, polarisation, spectral index and X-ray environment maps. Regions of the southern jet show simultaneous narrowing and brightening, which can be interpreted as a signature of the recollimation of the jet by external, environmental pressure, though it is also consistent with an intermittent Active Galactic Nuclei (AGN) or complex internal jet structure. X-ray data suggest that the environment is extremely poor; if the jet is indeed recollimating, the large recollimation scale (40 kpc) is consistent with a slow jet in a low-density environment.

astro-ph.GA↗

arXiv2022

Deep ASKAP EMU Survey of the GAMA23 field: Properties of radio sources

We present the Australian Square Kilometre Array Pathfinder (ASKAP) observations of the Galaxy and Mass Assembly (GAMA)-23h field. The survey was carried out at 887.5 MHz and covers a 83 square degree field. We imaged the calibrated visibility data, taken as part of the Evolutionary Mapping of Universe (EMU) Early Science Programme, using the latest version of the ASKAPSoft pipeline. The final mosaic has an angular resolution of 10 arcsec and a central rms noise of around 38 $μ$Jy beam$^{-1}$. The derived radio source catalogue has 39812 entries above a peak flux density threshold of 5$σ$. We searched for the radio source host galaxy counterparts using the GAMA spectroscopic (with an i-band magnitude limit of 19.2 mag) and multi-wavelength catalogues that are available as part of the collaboration. We identified hosts with GAMA spectroscopic redshifts for 5934 radio sources. We describe the data reduction, imaging, and source identification process, and present the source counts. Thanks to the wide area covered by our survey, we obtain very robust counts down to 0.2 mJy. ASKAP's exceptional survey speed, providing efficient, sensitive and high resolution mapping of large regions of the sky in conjunction with the multi-wavelength data available for the GAMA23 field, allowed us to discover 63 giant radio galaxies. The data presented here demonstrate the excellent capabilities of ASKAP in the pre-SKA era.

astro-ph.GA↗

arXiv2022

Searching for pulsars associated with polarised point sources using LOFAR: Initial discoveries from the TULIPP project

Discovering radio pulsars, particularly millisecond pulsars (MSPs), is important for a range of astrophysical applications, such as testing theories of gravity or probing the magneto-ionic interstellar medium. We aim to discover pulsars that may have been missed in previous pulsar searches by leveraging known pulsar observables (primarily polarisation) in the sensitive, low-frequency radio images from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS), and have commenced the Targeted search, using LoTSS images, for polarised pulsars (TULIPP) survey. For this survey, we identified linearly and circularly polarised point sources with flux densities brighter than 2 mJy in LoTSS images at a centre frequency of 144 MHz with a 48 MHz bandwidth. Over 40 known pulsars, half of which are MSPs, were detected as polarised sources in the LoTSS images and excluded from the survey. We have obtained beam-formed LOFAR observations of 30 candidates, which were searched for pulsations using coherent de-dispersion. Here, we present the results of the first year of the TULIPP survey. We discovered two pulsars, PSRs J1049+5822 and J1602+3901, with rotational periods of P=0.73 s and 3.7 ms, respectively. We also detected a further five known pulsars (two slowly-rotating pulsars and three MSPs) for which accurate sky positions were not available to allow a unique cross-match with LoTSS sources. This targeted survey presents a relatively efficient method by which pulsars, particularly MSPs, may be discovered using the flexible observing modes of sensitive radio telescopes such as the Square Kilometre Array and its pathfinders/precursors, particularly since wide-area all-sky surveys using coherent de-dispersion are currently computationally infeasible.

astro-ph.HE↗

arXiv2022

The LOFAR Two-metre Sky Survey -- V. Second data release

In this data release from the LOFAR Two-metre Sky Survey (LoTSS) we present 120-168MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44$^\circ$30' and 1h00m +28$^\circ$00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451hrs (7.6PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4,396,228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6" resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144MHz have: a median rms sensitivity of 83$μ$Jy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2"; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8mJy/beam. By creating three 16MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of +/-0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20" resolution 120-168MHz continuum images have a median rms sensitivity of 95$μ$Jy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 x 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8mJy/beam at 4' and 2.2mJy/beam at 20"; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data.

astro-ph.GA↗