arXiv subjects

M. Hoeft

Publications and source records attributed to M. Hoeft.

At least 19 recordsLinked to original sources

arXiv2026

The Large-Scale Structure of the Universe through the SKA lenses

The large-scale distribution of galaxies in the Universe forms an intricate, interconnected network known as the cosmic web. Cosmological simulations within the standard Lambda-CDM framework successfully reproduce this filamentary structure and predict that the nodes and filaments are filled with tenuous plasma at temperatures ranging from 10^5-10^8 K. The hottest and luminous plasma in the nodes corresponds to the intra-cluster medium, while the cooler, more tenuous, gas extends along filaments and cluster outskirts. Galaxies and galaxy groups form and flow along these filaments before accreting onto galaxy clusters (the nodes), outlining the dynamical evolution of large-scale structures. During this process, an enormous amount of energy is dissipated through complex plasma processes that can be traced by radio emitting electrons. Despite strong theoretical support for this picture, observational validation remains limited. While massive clusters have been widely detected across various wavelengths, cluster outskirts and the diffuse intergalactic medium within filaments has remained elusive due to their extremely faint emission. The advent of highly sensitive radio facilities such as LOFAR, uGMRT, and MeerKAT has recently enabled a few successful detections of emission from comparatively denser regions of the cosmic-web. These include radio megahalos, permeating the entire cluster volume, as well as bridges of radio emission connecting cluster pairs. In this chapter, we summarize current theoretical insights into the cosmic web, discuss observational strategies and recent discoveries, and highlight how the forthcoming Square Kilometre Array (SKA) is expected to transform our understanding of the cosmic web and the distribution of baryons in the Universe.

astro-ph.CO↗

arXiv2026

Galaxy clusters in the LoTSS-DR3: Catalogues and detection pipeline for diffuse radio emission

The third data release of the LOFAR Two-metre Sky Survey provides an unprecedented view of the northern sky at 144 MHz. 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. We aim to develop an automated pipeline to construct a catalogue of galaxy clusters hosting diffuse radio emission from LoTSS-DR3 20arcsec 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. 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. We produced a pixel-level segmentation map of the full LoTSS-DR3 and a quantitative indicator for the presence of diffuse 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 redshift of the clusters. [Abridged]

astro-ph.CO↗

arXiv2026

Variability of Sagittarius A* at 3 GHz on minute-scale with MeerKAT

The supermassive black hole Sagittarius A* (Sgr A*) exhibits temporal and spectral variability across the electromagnetic spectrum. However, variability at radio frequencies below ~ 5 GHz for timescales shorter than a day remains largely unexplored. We investigate the variability of Sgr A* at 2.79 GHz on short timescales (1 min), to probe an under-explored regime of its emission process. Through point-source model fitting in the uv-domain, we analyse the flux density variation of Sgr A* over an 8 h observation. We detect flux variation on a few tens of minute timescale with a modulation index of 6.11 %, a mean flux density of ($827 \pm 0.1_{\mathrm{stat}} \pm 33_{\mathrm{sys}}) \, \mathrm{mJy}$, and a mean spectral slope of $0.08\pm0.03$. Furthermore, we measure the slope of the structure function of the observed light curve as $0.81 \pm 0.05$ with a characteristic timescale of about 120 min. Our study at low radio frequencies is a critical step toward constraining the physical mechanisms that drive Sgr A*'s variable emission and its spectral energy distribution. Our study suggests that variability at centimetre and millimetre wavelengths is likely more closely related than previously thought.

astro-ph.GA↗

arXiv2026

The LOFAR sub-arcsecond view of the high-redshift radio relic in PSZ2G091.83+26.11

Enhanced inverse Compton (IC) losses at high redshift steepen diffuse radio spectra in galaxy clusters, making low-frequency (~100 MHz) observations favorable. However, low-frequency studies often lack the resolution needed to locate particle acceleration sites or separate diffuse emission from radio galaxies. In this paper, we unveil the properties of the radio relic in the distant cluster PSZ2G091.83+26.11 (z=0.822) by resolving the acceleration site and inspecting the downstream region. Using the European LOFAR (ILT) at 145 MHz, we study a radio relic at (sub-)arcsecond resolution for the first time below 1 GHz, complemented by arcsecond-resolution VLA data at higher frequencies. We confirm the diffuse emission is not a radio galaxy. A spectral index gradient toward the cluster center matches previous 5'' maps. High-resolution 0.4'' and 1.9'' images reveal emission ahead of the shock, connecting the relic to a radio galaxy. 1.9'' profiles across the downstream at 145 MHz and 3.0 GHz follow a log-normal magnetic field distribution. The 145 MHz shock surface shows a sharp discontinuity at the same location of a change in electron density, Rotation Measure, and fractional polarization, likely tied to magnetic field changes. Finally, we find hints of redshift evolution of the radio power versus cluster mass correlation. The impressive angular resolution achievable by the LOFAR long baselines is opening an unprecedented view of the low energetic plasma in galaxy clusters. This is extremely significant in the case of high-redshift clusters, where radio emission at low frequencies is less affected by energy losses but its detection is strongly limited by poor resolution.

astro-ph.CO↗

arXiv2026

The LOFAR Two-metre Sky Survey: VII. Third Data Release

We present the third data release of the LOFAR Two-metre Sky Survey (LoTSS-DR3). The survey images cover 88% of the northern sky and were created from 12,950 hrs of data (18.6 PB) accumulated over 10.5 years. The images were produced through direction-independent and direction-dependent calibration pipelines that correct for instrumental effects as well as spatially and temporally varying ionospheric distortions. In our 120-168 MHz continuum mosaic images with an angular resolution of 6 arcsec (9 arcsec below declination 10$^\circ$) we catalogue 13,667,877 sources, formed from 16,943,656 Gaussian components. The scatter in the astrometric precision approximately follows the expected noise-like behaviour but with an additional systematic component of at least 0.24 arcsec that is likely due to calibration imperfections. The random flux density scale error is 6%, while the systematic offset was previously shown to be within 2%. The median sensitivity of our mosaics is 92$\mu$Jy beam$^{-1}$. Completeness simulations, accounting for realistic source models, time- and bandwidth-smearing effects, and astrometric errors, indicate that we detect more than 95% of compact sources with integrated flux densities exceeding 9 times the local root mean square (RMS) noise. However, the recovered source counts in a particular integrated flux density bin do not match the injected counts until flux densities exceed 45 times the local RMS noise. The Euclidean-normalised differential source counts derived from the survey constrain the radio source population over five orders of magnitude and are in good agreement with previous deep and wide-area surveys. All data products are publicly available, including catalogues, individual-field Stokes I, Q, U, and V images, mosaicked Stokes I images, and $uv$ data with associated direction-dependent calibration solutions.

astro-ph.GA↗

arXiv2025

The longest known tails of ram-pressure-stripped star-forming galaxies are caused by an intracluster medium shock in Abell 1367

The environment plays an important role in shaping the evolution of cluster galaxies through mechanisms such as ram pressure stripping (RPS), whose effect may be enhanced in merging clusters. We investigate a complex of three galaxies UGC 6697, CGCG 097-073, and CGCG 097-079, that are currently undergoing extreme RPS, as evident from their multi-wavelength-detected tails. The galaxies are members of the nearby ($d=92$ Mpc) merging cluster Abell 1367 and are located in proximity to an intracluster medium (ICM) shock that is traced by X-ray observations and the presence of a radio relic. We analyzed LOFAR and MeerKAT observations at frequencies of 54, 144, 817, and 1270 MHz to perform a detailed spectral analysis of the tails. We found that all three tails are significantly more extended than in previous radio studies, with lengths of $\geq70$ kpc. For UGC 6697, we detected a tail of 300 kpc, making it the longest known RPS tail of a star-forming galaxy at any wavelength. The length and spectral variations of the tail cannot be explained purely by the spectral aging of stripped cosmic rays. We construct a model of the tail that includes compression and re-acceleration due to the encounter with the nearby ICM shock, which can plausibly account for the extreme RPS as well as the length and spectral variation of the tail. We further discover a radio plume at the leading edge of UGC 6697 that connects to a narrow filament. These sources exhibit extremely steep ($\alpha\approx-1.7$) and highly curved spectra. We speculate that this emission arises from cosmic rays re-energized by UGC 6697's rapid infall that propagate along magnetic filaments in the cluster center. Our findings represent direct evidence of a cluster merger shock impacting the evolution of member galaxies. Furthermore, we report the first tentative detection of particle acceleration at the leading edge of an infalling galaxy.

astro-ph.GA↗

arXiv2025

Combining the second data release of the European Pulsar Timing Array with low-frequency pulsar data

Low-frequency radio data improve the sensitivity of pulsar timing arrays (PTAs) to propagation effects such as dispersion measure (DM) variations, enabling better noise characterization essential for detecting the stochastic gravitational wave background (GWB). We combined LOFAR (100-200 MHz) and NenuFAR (30-90 MHz) observations with the recent European and Indian PTA release (DR2new+) into a new dataset, DR2low, spanning ~11 years for 12 pulsars. DR2low allows updated noise models, increasing PTA sensitivity to the GWB. Using Libstempo and Enterprise, we applied standard noise models including red noise (RN) and time-variable DM (DMv) as power laws, and performed Bayesian model selection over RN, DMv, and an additional chromatic noise term (CN4). Compared to DR2new+, DR2low improves DM constraints and separates DM and RN contributions. We found that the RN is required in the final model for 10 out of 12 pulsars, compared to only 5 in the DR2new+ dataset. The improved sensitivity to plasma effects provided by DR2low also favors the identification of significant CN4 in eight pulsars, while none showed such evidence in DR2new+. The analysis also reveals unmodelled solar wind effects, particularly near solar conjunction, with residual delays absorbed into the DM component, highlighting the importance of accurately modelling the solar wind in PTA datasets.

astro-ph.HE↗

arXiv2025

A view of the CIZA J2242.8+5301 galaxy cluster at very low radio frequencies

The galaxy cluster CIZA J2242.8+5301 is a well-studied merging galaxy cluster that hosts prominent double radio relics including the famous sausage relic, as well as other diffuse radio sources. Observations at frequencies below 100 MHz are essential for investigating the physics of radio relics as they provide unique access to the low-energy population of cosmic-ray electrons. We aim to study the morphology, spectral characteristics, and physical processes that produce relics. We present the first observations of the Sausage cluster at 45 MHz, the lowest radio frequency at which this cluster has been studied to date, using the Low Band Antenna (LBA) of the LOFAR radio interferometer. We made use of ten hours of LOFAR LBA observations, from which we achieved a thermal-noise limited radio image with a noise level of 1.5 mJy/beam at a resolution of 15 arcsec. These data were combined with existing multi-frequency measurements at higher frequencies: LOFAR High Band Antenna, Giant Metrewave Radio Telescope, Westerbork Synthesis Radio Telescope, and Karl G. Jansky Very Large Array. This broad frequency coverage allowed us to derive integrated spectral indices, spectral index and curvature maps, and Mach number distributions across the relics. We derived Mach numbers from the local injection index measure using low-frequency data with M_N = 2.9 +-0.5 for the northern relic and M_S = 2.9+-0.8 for the southern relic. LOFAR LBA observations reveal a remarkably symmetric surface brightness profile across the eastern part of the northern relic, with wings extending on either side of the peak. This discovery is contrary to the expectation of particle acceleration at a single, sharp shock and the subsequent downstream advection of accelerated electrons. We modelled the surface brightness profile, including the effects of projection, magnetic field variation, and shock deformation.

astro-ph.CO↗

arXiv2025

Radial Profiles of Radio Halos in Massive Galaxy Clusters: Diffuse Giants Over 2 Mpc

We present new, high frequency radio observations of the merging galaxy clusters PLCK G287.0+32.9, Abell 2744, and Bullet. These clusters are known to host $\sim$Mpc scale sources, known as radio halos, which are formed by the acceleration of cosmic rays by turbulence injected into the intracluster medium during cluster mergers. Our new images reveal previously undetected faint outermost regions of halos, extending to over 2 Mpc. This discovery highlights the presence of radio halos with large extents at high frequencies and suggests that their observable size depends on a combination of the observation sensitivity and uv-coverage, and their radio power. We additionally compare the properties of these three clusters with MACS J0717+3745 and Abell 2142, both of which are known to host prominent large radio halos. Remarkably, all five halos, despite their exceptionally large extents, exhibit properties similar to other classical halos: their radial profiles are described by a single-component exponential fit, they show radial spectral index steepening, and have an average radio emissivity of about $10^{-42}\, \mathrm{erg\,s^{-1}\,cm^{-3}\,Hz^{-1}}$. Our results demonstrate that radio halos can extend to the cluster periphery, without the transition to an observationally distinguishable different halo component in the outermost regions. Our findings highlight that careful subtraction of unrelated sources embedded in the halo is necessary to measure the radio surface brightness accurately, as incomplete subtraction can introduce an apparent secondary component in the peripheral regions.

astro-ph.CO↗

arXiv2025

Non-thermal filaments and AGN recurrent activity in the galaxy group Nest200047: a LOFAR, uGMRT, MeerKAT, VLA radio spectral analysis

Nest200047 is a clear example of multiple radio bubbles from an Active Galactic Nucleus (AGN) in a galaxy group, featuring non-thermal filaments likely shaped by buoyancy, gas motions, and stabilized by magnetic fields. This study presents high-quality data obtained from uGMRT, MeerKAT, and VLA, alongside existing LOFAR data, to analyze the system's morphology and spectrum over a broad frequency range (53-1518 MHz). Our findings reveal new filamentary emission in the inner 60 kpc, surrounding and extending from the inner bubbles and jets, suggesting complex dynamical evolution of the non-thermal plasma in the group core. The filaments have widths of a few kpc and lengths from tens to hundreds of kpc, with a steep and curved radio spectrum ($\rm \alpha=1\sim2$). They exhibit a constant spectral index profile along their length, implying particles are either (re-)accelerated together or move at super-Alfvenic speeds. Spectral aging analysis yields jet active times between 50 and 100 Myr with short inactive phases, suggesting continuous energy injection typical of AGN feedback in galaxy groups. This study highlights the potential of combining high-quality radio data to understand recurrent jet activity and feedback, with implications for future research with the SKA observatory.

astro-ph.GA↗

arXiv2024

The Jet Paths of Radio AGN and their Cluster Weather

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.

astro-ph.GA↗

arXiv2024

Mind the Gap between A2061 and A2067: Unveiling new diffuse large-scale radio emission

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.

astro-ph.CO↗

arXiv2024

Exploring the time variability of the Solar Wind using LOFAR pulsar data

High-precision pulsar timing is highly dependent on precise and accurate modeling of any effects that impact the data. It was 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 time scales. In this study, we test and validate a new, cutting-edge Solar wind modeling method included in the \texttt{enterprise} software suite through extended simulations, and we apply it to investigate temporal variability in LOFAR data. Our model testing scheme in itself provides an invaluable asset for pulsar timing array (PTA) experiments. As 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. We employ a Bayesian approach utilizing a continuously varying Gaussian process to model the solar wind referred to as Solar Wind Gaussian Process (SWGP). We conduct noise analysis on eight pulsars from the LOFAR dataset with most pulsars having a timespan of $\sim 11$ years encompassing one full solar activity cycle. Our analysis reveals a strong correlation between the electron density at 1 AU and the ecliptic latitude (ELAT) of the pulsar. Pulsars with $|ELAT|< 3^{\circ}$ exhibit significantly higher average electron densities. We observe distinct temporal patterns in electron densities in different pulsars. In particular, pulsars within $|ELAT|< 3^{\circ}$ exhibit similar temporal variations, while the electron densities of those outside this range correlate with the solar activity cycle. The continuous variability in electron density offered in this model represents a substantial improvement over previous models, which assume a single value for piece-wise bins of time. This advancement holds promise for solar wind modeling in future International Pulsar Timing Array data combinations.

astro-ph.SR↗

arXiv2023

A Collection of German Science Interests in the Next Generation Very Large Array

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.

astro-ph.IM↗

arXiv2023

A combined LOFAR and XMM-Newton analysis of the disturbed cluster PSZ2G113.91-37.01

In this work, we investigated 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 CHEX-MATE project, and the images of the 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 and one in the northern regions. The analysis of the intracluster medium distribution revealed the presence of a northern surface brightness jump associated to 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 of re-acceleration of particles in the northern relic, and we measured an associated Mach number of M = 1.95 $\pm$ 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 beta model to 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.

astro-ph.CO↗

arXiv2023

The LOFAR Two-Metre Sky Survey (LoTSS): VI. Optical identifications for the second data release

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.

astro-ph.GA↗

arXiv2023

The diffuse radio emission in the high-redshift cluster PSZ2 G091.83+26.11: total intensity and polarisation analysis with Very Large Array 1-4 GHz observations

We present the peculiar case of PSZ2G091.83+26.11 at z=0.822. This cluster hosts a Mpc-scale radio halo and an elongated radio source, whose location with the respect to the intracluster medium (ICM) distribution and to the cluster centre is not consistent with a simple merger scenario. We use VLA data at 1-4 GHz to investigate the spectral and polarisation properties of the diffuse radio emission. We combine them with previously published data from LOFAR n the 120-168 MHz band, and from the uGMRT at 250-500 and 550-900 MHz. We also complement the radio data with Chandra X-ray observations to compare the thermal and non-thermal emission of the cluster. The elongated radio emission is visible up to 3.0 GHz and has an integrated spectral index of $-1.24\pm0.03$, with a steepening from $-0.89\pm0.03$ to $-1.39\pm0.03$. These values correspond to Mach numbers $\mathcal{M}_{\rm radio,int}=3.0\pm0.19$ and $\mathcal{M}_{\rm radio,inj}=2.48\pm0.15$. Chandra data reveals a surface brightness discontinuity at the location of the radio source, with a compression factor of $\mathcal{C}=2.22^{+0.39}_{-0.30}$ (i.e. $\mathcal{M}_{\rm Xray}=1.93^{+0.42}_{-0.32}$). We also find that the source is polarised at GHz frequencies. We estimate an intrinsic polarisation fraction of $\sim0.2$, a Rotation Measure of $\sim50~{\rm rad~m^{-2}}$ (including the Galactic contribution) and an external depolarisation of $\sim60~{\rm rad~m^{-2}}$. The $B$-vectors are aligned with the major axis of the source, suggesting magnetic field compression. Hence, we classify this source as a radio relic. We also find a linear/super-linear correlation between the non-thermal and thermal emission. We propose an off-axis merger and/or multiple merger events to explain the position and orientation of the relic. Given the properties of the radio relic, we speculate that PSZ2G091.83+26.11 is in a fairly young merger state.

astro-ph.GA↗

arXiv2022

V-LoTSS: The Circularly-Polarised LOFAR Two-metre Sky Survey

We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We use the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly-polarised maps with median 140 $\mu$Jy beam$^{-1}$ noise and resolution of 20$''$ for $\approx$27% of the northern sky (5634 deg$^{2}$). The leakage of total intensity into circular polarisation is measured to be $\approx$0.06%, and our survey is complete at flux densities $\geq1$ 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$\pm$100 circularly-polarised sources.

astro-ph.GA↗