Andrea Botteon

The decametre radio sky (∼10–30 MHz) remains largely unexplored due to strong ionospheric distortions and severe radio-frequency interference. Despite its scientific potential, including access to steep-spectrum synchrotron emission from aged electron populations and coherent radio emission from planetary and stellar magnetospheres, sub-arcminute imaging at these frequencies has only recently become feasible with instruments such as the Low Frequency Array (LOFAR). In this work, we present the first data release (DR1) of the LOFAR decametre Sky Survey (LoDeSS), which maps the northern sky above a declination of 20° between 15 and 30 MHz. LoDeSS reaches an angular resolution of ∼45″ and typical noise levels of ∼11.5 mJy beam ⁻¹ at a central frequency of 23 MHz. Observations were obtained with the LOFAR Low Band Antenna (LBA) system using a multi-beam strategy, with typical integration times of ∼5 h per pointing. They were carried out at night to reduce the impact of ionospherically reflected radio-frequency interference. Data quality varies across fields as a result of the ionospheric conditions and the fraction of data flagged due to radio-frequency interference. This first release covers ∼15% of the full survey area and consists of fields originally selected around exoplanet candidates for radio auroral emission. The products released to the community include primary-beam-corrected images in Stokes I and V, and a source catalogue for fields with reliable flux density scales. This release demonstrates that LOFAR enables sensitive, wide-field imaging in the decametre regime with unprecedented resolution, opening new opportunities for studies of pulsars, exoplanetary systems, galaxy clusters, and other low-frequency phenomena.

We identified a peculiar bright and filamentary radio source at a low Galactic latitude ( b ≈ 0.5°) in LOFAR Two-meter Sky Survey data. This source, also detected in previous radio observations, was originally believed to be a pulsar, but was later suggested to lie in a heavily obscured galaxy cluster behind the Galactic plane. In this paper, we characterize the main properties of the host cluster (redshift, mass, temperature, X-ray luminosity, and dynamical status) for the first time, using X-ray observations from Chandra and SRG/eROSITA. In addition, we combined new uGMRT follow-up data with observations from the LOFAR LBA Sky Survey to perform a multifrequency, spatially resolved spectral analysis of the filamentary radio source (VLSS J0318.9+5755, hereafter referred to as the “Treble Clef” due to its morphology). We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the zone of avoidance. We speculate that its complex morphology is shaped by gas motions generated in the intracluster medium during the ongoing merger, which are also likely responsible for the candidate radio halo tentatively observed in the cluster center. Owing to its highly filamentary morphology, brightness at ≲1 GHz, and extremely steep spectrum, reaching values of α > 4 between 400 and 650 MHz, this source represents an ideal target for high-resolution, very-low-frequency follow-up observations with LOFAR2.0.

The emerging picture of radio haloes and mini-haloes in galaxy clusters is more blurred and far less distinct than once thought. While both trace cosmic ray electrons and magnetic fields in the intracluster medium (ICM), our knowledge of the mechanisms powering these sources, their properties and their connection to cluster dynamics, is incomplete. We present new deep MeerKAT L-band (856−1712 MHz) observations of the sloshing cool-core cluster 2A 0335+096, performed as part of the ‘MeerKAT-meets-LOFAR’ mini-halo census, as well as reprocessed LOFAR 120−168 MHz LOFAR Two-metre Sky Survey data. We present highly sensitive radio continuum images, from which we perform a spectral analysis and a comparison of the thermal and non-thermal properties using reprocessed archival XMM-Newton data. Our MeerKAT data reveal that the known mini-halo is embedded in a large-scale radio halo spanning up to 510 kpc in this sloshing cluster, although only the inner mini-halo is detected by LOFAR. We find a typical steep spectrum of α = −1.20 ± 0.09 for the mini-halo; the non-detection of the larger-scale halo by LOFAR indicates a spectral index no steeper than −1.2. Our results are broadly consistent with an interpretation of turbulent (re-)acceleration powered by sloshing within the cluster potential well. This work emphasises the need for using deep, high-dynamic multi-frequency radio observations in conjunction with high-quality X-ray observations to complete our understanding of the connection between radio halo properties and cluster dynamics.

We present a detailed, multiband radio continuum study of the compact galaxy group Stephan’s Quintet (HCG 92). We use a combination of new (MeerKAT, uGMRT) and archival (LOFAR, VLA) observations covering the 120 MHz–8 GHz frequency range to examine the radio properties of the group, focusing on the famous radio ridge and surrounding diffuse emission. We find filamentary substructure and branching in the southern half of the ridge, confirm an extension of the ridge to the northwest, and identify for the first time a radio counterpart to the gas bridge linking the ridge and NGC 7319. The northern ridge, northwest extension, and diffuse emission have relatively steep, curved spectra, with a high-frequency spectral index gradient running north–south along the ridge. We show that the ridge emission primarily arises from a single physical mechanism, probably strong ( M ≃ 40 − 100 ) shocks in cold gas, caused by the ∼850–1000 km s ⁻¹ collision between NGC 7318B and tidal gas filaments produced by past galaxy interactions. Synchrotron spectral age estimates suggest the collision began at the north end of the ridge ∼20 Myr ago, and finished only ∼5–6 Myr ago in the south, with the southern end of the shocked ridge likely still within or close to the disk of NGC 7318B. Based on this age gradient we find that the angle between the intruder galaxy’s motion and the tidal filaments was probably only ∼15°, and combining this with the lack of a spectral index gradient in the diffuse radio emission suggests that NGC 7318B’s direction of motion is probably close to the line of sight.

The galaxy cluster RXC J0032.1+1808 has been well-studied with optical imaging and gravitational lensing mass maps, both of which reveal an elongated morphology in the north–south direction. We find that its X-ray morphology is bimodal, suggesting that it is in the process of merging; combined with a previously reported detection of a radio relic, we suggest that the system is seen after first pericenter. We extract the global X-ray temperature and unabsorbed luminosity from archival XMM-Newton data, finding T X = 8 . 5 − 0.9 + 1.1 keV and L _X = 1.04 ± 0.03 × 10 ⁴⁵ erg s ⁻¹ at 90% confidence in the 0.5–10.0 keV energy range. We conduct a redshift survey of member galaxies and find that the line-of-sight relative velocity between the two subclusters is 1002–1785 km s ⁻¹ at 68% confidence, although the BCGs have an even larger difference of 2466 km s ⁻¹ . We use publicly available hydrodynamic simulations to show that it cannot be a head-on merger; it is consistent with an impact parameter of 0.5 or 1 Mpc. We also use the simulation set to find initial constraints on the time since pericenter (≈315–550 Myr) and the locus of possible lines of sight.

We present the LOFAR Galaxy Cluster Ultra-Deep Field, in which 336 h of LOFAR observations at 120–168 MHz have been collected on the nearby ( z = 0.080) cluster Abell 2255. This massive and merging system is known to host spectacular radio emission from both cluster galaxies and the intracluster medium. Previous LOFAR observations revealed pervasive diffuse synchrotron emission extending from the cluster center to its dynamically active outskirts, tracing relativistic electrons propagating in large-scale magnetic fields. In this work, we present a set of new ultra-deep images at the central frequency of 144 MHz based on the 224 h of data with the best quality, which reach a sensitivity of 24 µJy beam ⁻¹ at 7.1″×4.3″ resolution. These images represent the deepest radio observations of a galaxy cluster obtained to date and provide a glimpse of what should be routinely observed in clusters with SKA-Low in the near future. Using these data, we investigate the topology of the cluster magnetic field out to its virial radius by applying the synchrotron intensity gradient technique. We find that the inferred magnetic field exhibits preferential orientations in distinct regions of the cluster, such as in radio halo extensions (bridges) and in relics, suggesting that the dynamics of the cluster formation process is shaping the large-scale magnetic field. This interpretation is supported by the comparison with the magnetic field orientation obtained from cosmological magnetohydrodynamic simulations. This work provides the first indication of a coherent, large-scale magnetic field topology across an entire galaxy cluster, from the core to the outskirts, and it demonstrates the unique power of ultra-deep, low-frequency observations to trace the structure of cluster magnetic fields on megaparsec scales, thereby probing the magnetization of the large-scale structure of the Universe.

We present the first detailed numerical modeling of the radio emission from MACS J0018.5+1626 as part of the Improved Constraints on Mergers with Sunyaev–Zel’dovich (SZ), Hydrodynamical Simulations, Optical, and X-ray (ICM-SHOX) project. By matching X-ray, thermal and kinetic SZ, optical, and lensing observables to simulations, the ICM-SHOX pipeline indicates that MACS J0018.5+1626 is undergoing a binary merger close to pericenter passage and is observed along a line of sight nearly aligned with the merger axis. We perform 3D magnetohydrodynamics simulations of binary cluster mergers coupled to tracer particles and a Fokker–Planck solver to model the radio emission. Exploring variations in the most likely initial conditions within the ICM-SHOX parameter space, such as the relative cluster velocity and impact parameter, we find that the resulting merger configuration consistently produces two merger-driven shocks with typical average Mach numbers M s ∼ 2 –3 with corresponding standard deviations of σ M ∼ 0.5 –1.5. Within this framework, we examine the cluster conditions under which standard diffusive shock acceleration can reproduce LOFAR observations. In particular, we discuss the possibility that the apparent radio halo seen by LOFAR arises from the superposition of two radio relics viewed nearly face on.

Context. The third data release (DR3) of the LOFAR Two-metre Sky Survey (LoTSS) provides an unprecedented view of the northern sky at 144 MHz, containing more than 13 million radio sources. While compact sources can be efficiently identified with automated software packages, the detection of diffuse radio emission associated with galaxy clusters still requires dedicated processing and visual inspection. Given the scale of current and forthcoming radio surveys, automated approaches based on artificial intelligence are becoming essential to the identification of the most interesting targets. Aims. We aim to develop an automated pipeline to construct a catalogue of galaxy clusters hosting diffuse radio emission from LoTSS-DR3 20″ images. The pipeline is designed to provide both the probability that a cluster hosts diffuse radio emission and an interpretable image of its shape and morphology. Methods. We employed Radio U-Net, a convolutional neural network optimised for image segmentation (i.e. pixel-level identification) of diffuse radio emission. To associate detected emission with individual clusters, we combined the network output with positional, mass, and redshift information from four X-ray- and Sunyaev–Zeldovich-selected cluster catalogues, resulting in a merged sample of 3822 clusters covered by the LoTSS-DR3. Results. We produced a pixel-level segmentation map of the full LoTSS-DR3 and a quantitative indicator (ℛ value) for the presence of diffuse radio emission in each cluster. This enables the selection of sub-samples with specific properties for targeted follow-up or statistical studies. As a demonstration of the first application, we identified a sub-sample of 357 clusters selected at the highest network accuracy (76%), and we showed some examples of newly detected systems. For the second, using a larger statistical sample, we verified that the detection fraction of diffuse radio sources in the four catalogues increases with the mass and decreases with redshift of the clusters. Conlcusions. This work establishes a flexible and scalable framework for deep learning-based searches for diffuse radio emission in new-generation wide-area radio surveys.

Context. Radio relics are diffuse, extended synchrotron sources located in the outskirts of merging galaxy clusters. Their origin has been linked to shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front remains under debate. Some clusters, such as MACS J1752.0+4440, host a double relics system, with two relics located on opposite sides of the cluster center. Aims. To investigate the acceleration mechanism that generates radio relics, we studied the morphological and spectral properties of double relic system in MACS J1752.0+4440 ( z = 0.366). Methods. We present new wideband radio continuum observations obtained with uGMRT and JVLA, and previously published LOFAR data. We performed a detailed, high-resolution spectral analysis of the double relic system in MACS J1752.0+4440, and we observed and characterized substructures, particularly those of the brighter relic. Results. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a “bright bar” substructure. Moreover, we observe surprisingly flat integrated spectral indices for both relics, at α _int ^NE = −0.91 ± 0.06 and α _int ^SW = −0.83 ± 0.05. We studied the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate injection Mach numbers of ℳ _NE ^SW = 3.1 ^+0.1 _−0.1 and ℳ _NE ^SW = 3.2 ^+0.1 _−0.1 . We performed a spectral curvature analysis for both relics, generating color-color plots, and we observe two “concave” spectra characterized by positive spectral curvature, in contrast to particle population aging models. Conclusions. The observed properties of the NE relic are not consistent with a simple scenario involving a single shock front and a single particle-injection event. Multiple shock surfaces, reacceleration, and projection effects likely play a role in shaping the morphology of the relic.