We studied bent radio sources within X-ray galaxy groups in the COSMOS and XMM-LSS fields, using radio data from the MeerKAT International GHz Tiered Extragalactic Explorations data release 1 (MIGHTEE-DR1) at 1.2-1.3 GHz (angular resolutions of 8.9" and 5"; 3.5 and 5.5 uJy/beam). Bent radio active galactic nuclei (AGN) were identified via visual inspection. Our analysis included 19 bent radio AGN in the COSMOS field and 17 in the XMM-LSS field which lie within X-ray galaxy groups (2x10^13 >= M200c/Msun = 3x10^14). We investigated the relationship between their bending angle (BA) - the angle formed by the jets or lobes of two-sided radio sources associated with AGN - and properties of their host galaxies and large-scale environment probed by the X-ray galaxy groups. Our key findings are: a) In the XMM-LSS field, we observed a strong correlation between the linear projected size of the bent AGN, the group halo mass, and the projected distance from the group centre. This trend, consistent with previous studies, was not detected in the COSMOS sample. b) The BA is a function of environmental density, with the type of medium playing a significant role. Additionally, at z <= 0.5 we found a higher number of bent sources (BA <= 160deg) compared to higher redshifts (z 1), by a factor of >1.5. This trend aligns with magnetohydrodynamic simulations, which suggest that denser environments and longer interaction times at lower redshifts contribute to this effect. Comparison with the literature suggests that jet bending in galaxy groups within the redshift range 0.1 < z < 1.2 is primarily driven by ram pressure exerted on the jets, which occurs during quiescent phases of AGN activity. This study underscores the role of environmental interactions in shaping the morphology of radio AGN within galaxy groups, providing insights into the interplay between large-scale structure and AGN physics.

The clusters Abell 2061 and Abell 2067 in the Corona Borealis supercluster have been studied at different radio frequencies and are both known to host diffuse radio emission. The aim of this work is to investigate the radio emission in between them, suggested by low resolution observations. We analyse deep LOFAR HBA observations at 144 MHz to follow up on the possible intercluster filament suggested by previous 1.4 GHz observations. We investigate the radial profiles and the point-to-point surface brightness correlation of the emission in A2061 with radio and Xray observations, to describe the nature of the diffuse emission. We report the detection of diffuse radio emission on 800 kpc scale, more extended than previously known, reaching beyond the radio halo in A2061 towards A2067 and over the separation outside the two clusters R500 radii. We confirm the presence of a radio halo in A2061, while do not find evidence of diffuse emission in A2067. The surface brightness profile from the centre of A2061 shows an excess of emission with respect to the azimuthally averaged radio halo profile and X-ray background. We explore three different dynamical scenario to explain the nature of the diffuse emission. We analyse a trail of emission of 760 kpc between the radio halo and radio relic in A2061. This pre merger system closely resembles the two other cluster pairs where radio bridges connecting the radio halos on Mpc scales have been detected. The diffuse emission extends beyond each cluster R500 radius but in this unique case, the absence of the radio halo in A2067 is likely the reason for the observed 'gap' between the two systems. However, the point-to-point correlation results are challenging to explain. The classification of the emission remains unclear, and detailed spectral analysis and further Xray observations are required to understand the origin of the diffuse emission.

Context. High-precision pulsar timing is highly dependent on the precise and accurate modelling of any effects that can potentially impact the data. In particular, effects that contain stochastic elements contribute to some level of corruption and complexity in the analysis of pulsar-timing data. It has been shown that commonly used solar wind models do not accurately account for variability in the amplitude of the solar wind on both short and long timescales. Aims. In this study, we test and validate a new, cutting-edge solar wind modelling method included in the enterprise software suite (widely used for pulsar noise analysis) through extended simulations. We use it to investigate temporal variability in LOFAR data. Our model testing scheme in itself provides an invaluable asset for pulsar timing array (PTA) experiments. Since, improperly accounting for the solar wind signature in pulsar data can induce false-positive signals, it is of fundamental importance to include in any such investigations. Methods. We employed a Bayesian approach utilising a continuously varying Gaussian process to model the solar wind. It uses a spherical approximation that modulates the electron density. This method, which we refer to as a solar wind Gaussian process (SWGP), has been integrated into existing noise analysis software, specifically enterprise. Our Validation of this model was performed through simulations. We then conduct noise analysis on eight pulsars from the LOFAR dataset, with most pulsars having a time span of similar to 11 years encompassing one full solar activity cycle. Furthermore, we derived the electron densities from the dispersion measure values obtained by the SWGP model. Results. Our analysis reveals a strong correlation between the electron density at 1 AU and the ecliptic latitude (ELAT) of the pulsar. Pulsars with |ELAT|< 3 degrees exhibit significantly higher average electron densities. Furthermore, we observed distinct temporal patterns in the electron densities in different pulsars. In particular, pulsars within |ELAT|< 3 degrees exhibit similar temporal variations, while the electron densities of those outside this range correlate with the solar activity cycle. Notably, some pulsars exhibit sensitivity to the solar wind up to 45 degrees away from the Sun in LOFAR data. Conclusions. The continuous variability in electron density offered in this model represents a substantial improvement over previous models, that assume a single value for piece-wise bins of time. This advancement holds promise for solar wind modelling in future International Pulsar Timing Array (IPTA) data combinations.

Context. Galaxy cluster mergers are believed to generate large-scale shock waves that are ideal sites for cosmic ray production. In these so-called radio relic shocks, synchrotron radiation is produced mainly as a result of electron acceleration in the presence of intracluster magnetic fields. Aims. We aim to compute radio emission light curves for a sample of galaxy group and cluster mergers simulated in a cosmological context in order to study the dependence of radio luminosity on cluster mass, redshift, and impact parameter. Methods. We used model galaxy clusters from THE THREE HUNDRED project, a sample of 324 simulated high-density regions of radius 15 h(-1) Mpc extracted from a cosmological volume, to identify cluster mergers characterised by the two main merging structures, construct their light curves, and follow their evolution throughout the complete simulated cosmic history. Results. We found that the median non-thermal radio relic luminosity light curve produced in galaxy cluster mergers can be described by a skewed Gaussian function abruptly rising after core-passage of the secondary cluster that peaks after similar to 0.1-0.8 Gyr as a function of M-200,M-1, the mass of the primary, displaying a mass-dependent luminosity output increase of less than or similar to 10 to about greater than or similar to 10-50 times relative to the radio emission measured at core-passage for galaxy groups and clusters, respectively. In general, most merger orbits are fairly radial with a median opening angle of similar to 20 degrees before the collision. We also found that, independent of the cluster mass, less radial mergers tend to last longer, although the trend is weak. Finally, in agreement with previous works, we found that the peak radio luminosity shows a significant correlation with mass, P-1.4 proportional to M-200,1(2.05) demonstrating that this relation holds all the way up from galaxy group scales to the most massive galaxy clusters. Conclusions. We conclude that cluster mass is the primary driver for radio 'gischt' median luminosity, although there are significant variations for a given cluster mass related to the specifics of each merging process. In general, binary mergers are responsible for many of the well-known observed radio relic structures but complex situations involving three or more substructures are also common. Our simulations suggest that the shock-driven, non-thermal radio emission observed on cluster outskirts are the result of massive galaxy cluster mergers at z less than or similar to 1, peaking at z similar to 0-0.5.

In this work, we investigate the interplay between the X-ray and radio emission of the cluster PSZ2G113.91-37.01 ( z = 0.371) using the high-quality XMM-Newton observations of the Cluster HEritage project with XMM-Newton – Mass Assembly and Thermodynamics at the Endpoint of structure formation (CHEX-MATE), and the images from the second data release of the LOFAR Two-meter Sky Survey (LoTSS-DR2). The cluster is undergoing a merger along the north-south axis and shows a central radio halo and two radio relics, one in the southern region and one in the northern one. Analysis of the intracluster medium (ICM) distribution revealed the presence of a northern surface brightness (SB) jump associated with the merger event. By extracting spectra across this discontinuity, we classified the edge as a cold front. Furthermore, we made use of upgraded Giant Metrewave Radio Telescope observations that allowed us to perform a spectral analysis of the G113 radio emission. We found evidence for the re-acceleration of particles in the northern relic, and we measured an associated Mach number of ℳ = 1.95 ± 0.01, as inferred from radio observations. We then performed a point-to-point analysis of the X-ray and radio emission, both in the halo and in the northern relic regions. We found a strong correlation for the halo and an anti-correlation for the relic. The former behaviour is in agreement with previous studies. The relic anti-correlation is likely related to the reverse radial distribution of the X-ray (increasing towards the cluster centre) and radio (decreasing towards the cluster centre) emissions. Finally, we performed a point-to-point analysis of the radio emission and the residuals obtained by subtracting a double β model from the X-ray emission. We found a strong correlation between the two quantities. This behaviour suggests the presence of a connection between the process responsible for the radio emission and the one that leaves fluctuations in the X-ray observations.

Context. Diffuse radio emission in galaxy clusters, namely radio halos and radio relics, is usually associated with merger events. Despite the tremendous advances in observations in the last decades, the particle (re-)acceleration and magnetic field amplification mechanisms and the connection with the stage and geometry of the cluster merger are still uncertain. Aims. In this paper, we present the peculiar case of PSZ2 G091.83+26.11 at z = 0.822. This cluster hosts a megaparsec-scale radio halo and an elongated radio source whose morphology resembles that of a radio relic. However, the location of this diffuse radio source with respect to the intracluster medium (ICM) distribution and to the cluster centre is not consistent with a simple merger scenario. Methods. We use Karl Jansky Very Large Array data at 1–4 GHz to investigate the spectral and polarisation properties of the diffuse radio emission. We combine these data with previously published data from the Low Frequency Array (LOFAR) in the 120–168 MHz band and from the upgraded Giant Metrewave Radio Telescope (uGMRT) at 250–500 and 550–900 MHz. Finally, we complement the radio data with Chandra X-ray observations in order to compare the thermal and non-thermal emission of the cluster. Results. The elongated radio emission east of the cluster is visible up to 3.0 GHz and has an integrated spectral index of α144 MHz3.0 GHz = −1.24 ± 0.03, with a steepening from −0.89 ± 0.03 to −1.39 ± 0.03. These values correspond to Mach numbers ℳradio, int = 3.0 ± 0.19 and ℳradio, inj = 2.48 ± 0.15. Chandra data revealed a surface brightness discontinuity at the location of the radio source with a compression factor of C = 2.22 −0.30+0.39 (i.e. MXray = 1.93−0.32+0.42). We also found that the source is polarised at GHz frequencies. Using QU-fitting, we estimate an intrinsic polarisation fraction of p0 ∼ 0.2, a Rotation Measure of ∼50 rad m−2 (including the Galactic contribution), and an external depolarisation of σRM ∼ 60 rad m−2. The polarisation B-vectors are aligned with the major axis of the source, suggesting magnetic field compression. Hence, we classify this source as a radio relic. Finally, we found a trend consistent with a linear or super-linear correlation between the non-thermal and thermal emission. Conclusions. We propose an off-axis merger and/or multiple merger events to explain the position and orientation of the relic with the respect to the ICM emission. Given the properties of the radio relic, we speculate that PSZ2 G091.83+26.11 is in a fairly young merger state.

The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27% of the northern sky, with a total area of ~5700 deg1. The high angular resolution of LOFAR with Dutch baselines (6 arcsec) allows us to carry out optical identifications of a large fraction of the detected radio sources without further radio followup; however, the process is made more challenging by the many extended radio sources found in LOFAR images as a result of its excellent sensitivity to extended structure. In this paper we present source associations and identifications for sources in the second data release based on optical and near-infrared data, using a combination of a likelihood-ratio cross-match method developed for our first data release, our citizen science project Radio Galaxy Zoo: LOFAR, and new approaches to algorithmic optical identification, together with extensive visual inspection by astronomers. We also present spectroscopic or photometric redshifts for a large fraction of the optical identifications. In total 4 116 934 radio sources lie in the area with good optical data, of which 85% have an optical or infrared identification and 58% have a good redshift estimate. We demonstrate the quality of the dataset by comparing it with earlier optically identified radio surveys. This is by far the largest ever optically identified radio catalogue, and will permit robust statistical studies of star-forming and radio-loud active galaxies.

We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We used the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly polarised maps with a median noise of 140 µJy beam −1 and resolution of 20″ for ≈27% of the northern sky (5634 deg 2 ). The leakage of total intensity into circular polarisation is measured to be ≈0.06%, and our survey is complete at flux densities ≥1 mJy. A detection is considered reliable when the circularly polarised fraction exceeds 1%. We find the population of circularly polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. The stellar systems can be further separated into chromospherically active stars, M dwarfs, and brown dwarfs. Based on the circularly polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. By the completion of this survey of the northern sky, we expect to detect 300±100 circularly polarised sources.

The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27% of the northern sky, with a total area of $\sim 5,700$ deg$^2$. The high angular resolution of LOFAR with Dutch baselines (6 arcsec) allows us to carry out optical identifications of a large fraction of the detected radio sources without further radio followup; however, the process is made more challenging by the many extended radio sources found in LOFAR images as a result of its excellent sensitivity to extended structure. In this paper we present source associations and identifications for sources in the second data release based on optical and near-infrared data, using a combination of a likelihood-ratio cross-match method developed for our first data release, our citizen science project Radio Galaxy Zoo: LOFAR, and new approaches to algorithmic optical identification, together with extensive visual inspection by astronomers. We also present spectroscopic or photometric redshifts for a large fraction of the optical identifications. In total 4,116,934 radio sources lie in the area with good optical data, of which 85% have an optical or infrared identification and 58% have a good redshift estimate. We demonstrate the quality of the dataset by comparing it with earlier optically identified radio surveys. This is by far the largest ever optically identified radio catalogue, and will permit robust statistical studies of star-forming and radio-loud active galaxies.

The Next Generation Very Large Array (ngVLA) is a planned radio interferometer providing unprecedented sensitivity at wavelengths between 21 cm and 3 mm. Its 263 antenna element array will be spatially distributed across North America to enable both superb low surface brightness recovery and sub-milliarcsecond angular resolution imaging. The project was developed by the international astronomy community under the lead of the National Radio Astronomy Observatory (NRAO), and is anticipated to be built between 2027 and 2037. Two workshops have been held in 2022 and 2023 with the goal to discuss and consolidate the scientific interests in the ngVLA within the German astronomical community. This community paper constitutes a collection of 48 science ideas which the German community aims to pursue with the ngVLA in the 2030s. This is not a complete list and the ideas are not developed at the level of a "Science Book", such that the present document is mainly meant provide a basis for further discussion within the community. As such, additional contributions are welcome, and will be considered for inclusion in future revisions.

We present the first detailed analysis of the radio halo in the merging galaxy cluster Abell 2256 using the LOw Frequency ARray, the upgraded Giant Metrewave Radio Telescope, and the Karl G. Jansky Very Large Array. Radio observations (120 MHz–2 GHz) combined with archival Chandra and XMM-Newton X-ray data allowed us to study the central radio halo emission with unprecedented detail. The integrated radio emission from the entire halo is characterized by an ultra-steep spectrum, which can be described by a power law with α 144 MHz 1.5 GHz = −1.63 ± 0.03 and radial steepening in the outer regions. The halo is significantly underluminous according to the current scaling relations between radio power and mass at 1.4 GHz, not at 150 MHz; ultra-steep spectrum halos are predicted to be statistically underluminous. Despite the complex structure of this system, the halo morphology is remarkably similar to that of the X-ray emission. The radio surface brightness distribution across the halo is strongly correlated with the X-ray brightness of the intracluster medium. The derived correlations show sublinear slopes and distinct structures: the core is I R ∝ I X 1.51 , the outermost region I R ∝ I X 0.41 , and we find radio morphological connections with X-ray discontinuities. We also find a strong anticorrelation between the radio spectral index and the X-ray surface brightness, implying radial steepening. We suggest that the halo core is either related to old plasma from previous active galactic nuclei activity, being advected, compressed, and reaccelerated by mechanisms activated by the cold front or less turbulent with strong magnetic field in the core. The change in the radio versus X-ray correlation slopes in the outer regions of the halo could be due to a radial decline of the magnetic field, the increase in the number density of seed particles, or increasing turbulence. Our findings suggest that the emitting volume is not homogenous according to turbulent reacceleration models.

We present wideband polarimetric observations, obtained with the Karl G. Jansky Very Large Array (VLA), of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We use both Rotation Measure Synthesis and QU-fitting, and find a reasonable agreement of the results obtained with these methods, in particular, when the Faraday distribution is simple and the depolarization is mild. The relic is highly polarized over its entire length reaching a fractional polarization ${>}30\%$ in some regions. We also observe a strong wavelength-dependent depolarization for some regions of the relic. The northern part of the relic shows a complex Faraday distribution suggesting that this region is located in or behind the intracluster medium (ICM). Conversely, the southern part of the relic shows a Rotation Measure very close to the Galactic foreground, with a rather low Faraday dispersion, indicating very little magnetoionic material intervening the line-of-sight. From spatially resolved polarization analysis, we find that the scatter of Faraday depths correlates with the depolarization, indicating that the tangled magnetic field in the ICM causes the depolarization. At the position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of two components clearly separated in Faraday space. The high Faraday dispersion component seems to be associated with the NAT, suggesting the NAT is embedded in the ICM while the southern part of the relic lies in front of it. The magnetic field orientation follows the relic structure indicating a well-ordered magnetic field. We also detect polarized emission in the halo region; however the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission, previously considered as the part of the halo, has a shock(s) origin.

The presence of large-scale magnetic fields and ultra-relativistic electrons in the intra-cluster medium (ICM) is confirmed through the detection of diffuse radio synchrotron sources, so-called radio halos and relics. Due to their steep-spectrum nature, these sources are rarely detected at frequencies above a few GHz, especially in low-mass systems. The aim of this study is to discover and characterise diffuse radio sources in low-mass galaxy clusters in order to understand their origin and their scaling with host cluster properties. We searched for cluster-scale radio emission from low-mass galaxy clusters in the Low Frequency Array (LOFAR) Two-metre Sky Survey - Data Release 2 (LoTSS-DR2) fields. We made use of existing optical (Abell, DESI, WHL) and X-ray (comPRASS, MCXC) catalogues. The LoTSS-DR2 data were processed further to improve the quality of the images that are used to detect and characterize diffuse sources. We have detected diffuse radio emission in 28 galaxy clusters. The number of confirmed (candidates) halos and relics are six (seven) and 10 (three), respectively. Among these, 11 halos and 10 relics, including candidates, are newly discovered by LOFAR. Beside these, five diffuse sources are detected in tailed radio galaxies and are probably associated with mergers during the formation of the host clusters. We are unable to classify other 13 diffuse sources. We compare our newly detected, diffuse sources to known sources by placing them on the scaling relation between the radio power and the mass of the host clusters.

We present deep and high-fidelity images of the merging galaxy cluster A2256 at low frequencies using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics with a remarkably spectacular network of filamentary substructures. The new uGMRT (300–850 MHz) and LOFAR (120–169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1–4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6″ resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of −1.07 ± 0.02 between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re)acceleration of electrons that are responsible for the observed radio emission.

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of $\geq$ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 degrees and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 arcsec, for sources observed in the survey. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120-168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44 degrees 30' and 1h00m +28 degrees 00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) 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 144 MHz have: a median rms sensitivity of 83 mu Jy beam(-1); 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 0.8 mJy beam(-1). By creating three 16 MHz 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 120-168 MHz continuum images have a median rms sensitivity of 95 mu Jy beam(-1), and we estimate a Stokes Ito 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 10.8 mu Jy beam(-1) at 4' and 2.2 mu Jy beam(-1) 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 to facilitate the thorough scientific exploitation of this unique dataset.

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3 x 0.2 although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.

Abell 3266 is a massive and complex merging galaxy cluster that exhibits significant substructure. We present new, highly sensitive radio continuum observations of Abell 3266 performed with the Australian Square Kilometre Array Pathfinder (0.8-1.1 GHz) and the Australia Telescope Compact Array (1.1-3.1 GHz). These deep observations provide new insights into recently-reported diffuse non-thermal phenomena associated with the intracluster medium, including a 'wrong-way' relic, a fossil plasma source, and an as-yet unclassified central diffuse ridge, which we reveal comprises the brightest part of a large-scale radio halo detected here for the first time. The 'wrong-way' relic is highly atypical of its kind: it exhibits many classical signatures of a shock-related radio relic, while at the same time exhibiting strong spectral steepening. While radio relics are generally consistent with a quasi-stationary shock scenario, the 'wrong-way' relic is not. We study the spectral properties of the fossil plasma source; it exhibits an ultra-steep and highly curved radio spectrum, indicating an extremely aged electron population. The larger-scale radio halo fills much of the cluster centre, and presents a strong connection between the thermal and non-thermal components of the intracluster medium, along with evidence of substructure. Whether the central diffuse ridge is simply a brighter component of the halo, or a mini-halo, remains an open question. Finally, we study the morphological and spectral properties of the multiple complex radio galaxies in this cluster in unprecedented detail, tracing their evolutionary history.

We present the first detailed analysis of the radio halo in the merging galaxy cluster Abell 2256 using the LOw Frequency ARray, the upgraded Giant Metrewave Radio Telescope, and the Karl G. Jansky Very Large Array. Radio observations (120 MHz-2 GHz) combined with archival Chandra and XMM-Newton X-ray data allowed us to study the central radio halo emission with unprecedented detail. The integrated radio emission from the entire halo is characterized by an ultra-steep spectrum, which can be described by a power law with alpha(1.5 GHz )(144 MHz) = -1.63 +/- 0.03 and radial steepening in the outer regions. The halo is significantly underluminous according to the current scaling relations between radio power and mass at 1.4 GHz, not at 150 MHz; ultra-steep spectrum halos are predicted to be statistically underluminous. Despite the complex structure of this system, the halo morphology is remarkably similar to that of the X-ray emission. The radio surface brightness distribution across the halo is strongly correlated with the X-ray brightness of the intracluster medium. The derived correlations show sublinear slopes and distinct structures: the core is I-R proportional to I (1.51)(X), the outermost region I-R proportional to I-0.41 (X) , and we find radio morphological connections with X-ray discontinuities. We also find a strong anticorrelation between the radio spectral index and the X-ray surface brightness, implying radial steepening. We suggest that the halo core is either related to old plasma from previous active galactic nuclei activity, being advected, compressed, and reaccelerated by mechanisms activated by the cold front or less turbulent with strong magnetic field in the core. The change in the radio versus X-ray correlation slopes in the outer regions of the halo could be due to a radial decline of the magnetic field, the increase in the number density of seed particles, or increasing turbulence. Our findings suggest that the emitting volume is not homogenous according to turbulent reacceleration models.