arXiv subjects

S. Giacintucci

Publications and source records attributed to S. Giacintucci.

At least 19 recordsLinked to original sources

arXiv2026

Extended radio emission in and around the merging cluster A520 as seen by MeerKAT

We present results from a deep MeerKAT 1.28 GHz observation of Abell 520, a galaxy cluster in the middle of a merger and a treasure trove of merger-generated phenomena. MeerKAT's combination of high angular resolution and sensitivity to extended radio emission lets us uncover unprecedented detail of the cluster's giant halo and puzzling features beyond the halo. We find that the main radio halo is sharply bounded along the direction of the NE-SW merger axis by the two X-ray shock fronts. In the perpendicular direction, the halo wings smoothly span over 2 Mpc, with the spectrum steepening in the outskirts. We detect faint radio emission upstream of the main (SW) bow shock; the emissivity jump across this shock can be used to constrain the physical mechanisms at work there. We discover a tail of radio emission extending past the NE shock to the gas-poor, high-entropy ``dark subcluster'' in the NE outskirts. The tail and subcluster have a higher radio/X-ray brightness ratio than the main halo's, suggesting efficient reacceleration of cosmic rays. We also discover two peripheral radio relics beyond the cluster virial radius, and a faint, $\sim2$ Mpc long, broad radio bridge connecting the dark subcluster to one of the relics along the direction perpendicular to the merger axis. The spectral slopes of the dark subcluster and the bridge are consistent with that of the main halo. This bridge and both relics may be caused by another giant merger-generated shock front at the virial radius.

astro-ph.CO↗

arXiv2026

A radio continuum view of Stephan's Quintet: age, dynamics and origin of the shock

We present a detailed, multi-band 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 $\rm 120\,MHz{-}8\,GHz$ 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 ($\mathcal{M}\simeq 40-100$) shocks in cold gas, caused by the $\rm \sim850-1000\,kms^{-1}$ 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 $\rm \sim20\,Myr$ ago, and finished only $\rm \sim5-6\,Myr$ 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 about 15 degrees, 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.

astro-ph.GA↗

arXiv2026

Radio Halos in Galaxy Clusters as unveiled by the SKA telescope

Giant radio halos (RHs) are diffuse, Mpc-scale synchrotron sources observed in a growing fraction of galaxy clusters. They trace relativistic particles and magnetic fields in the intracluster medium (ICM), providing a unique window into non-thermal processes and their role in cluster evolution. RHs are primarily found in merging systems, supporting models in which turbulence generated during cluster collisions re-accelerates pre-existing electrons to the energies required for the observed radio emission. In this scenario, the occurrence, power, and spectral properties of RHs depend on the energetics of cluster mergers, with the most massive and dynamically disturbed clusters hosting the most powerful halos. Low-frequency observations are crucial to uncover ultra-steep-spectrum RHs, a key prediction of turbulent re-acceleration models, and are expected to arise from less energetic merger events. LOFAR has enabled statistical studies of large cluster samples, placing robust constraints on RH occurrence and spectral trends. In this Chapter, we model RH formation and evolution using Monte Carlo simulations calibrated on LoTSS-DR2 findings, and we present predictions for SKA-Low in the AA4 configuration. Our results show that SKA will probe an unprecedented region of cluster mass and redshift space, detecting at least $\sim 2500$ RHs up to $z \approx 0.6$, including $\gtrsim 1000$ ultra-steep-spectrum systems, and revealing halos in clusters down to $\sim 10^{14}\, M_\odot$ and out to $z \approx 1$. These surveys will provide stringent tests of turbulent re-acceleration models and significantly advance our understanding of non-thermal processes in galaxy clusters.

astro-ph.CO↗

arXiv2026

A MeerKAT view of the Neutral Atomic Gas in Stephan's Quintet

We present new MeerKAT 21cm spectral line observations of the neutral hydrogen gas in the compact galaxy group Stephan's Quintet (HCG 92). These data provide a significantly improved view of the atomic gas distribution and kinematics in the group. New features include the first detections of HI associated with member galaxies NGC 7319 and NGC 7320C, the identification of an additional high-velocity HI component associated with SQ-A, and the detection of additional HI at low velocities filling much of the area of the NGC~7318B disk. We also find HI in the previously detected gas bridge linking NGC 7319 and NGC 7318B, and a new northern bridge linking NGC 7319 to the SQ-A star-formation region. We detect HI with a wide range of velocities along the line of sight through the northern half of the famous shock ridge, including in the 6200-6500$\rm\,km\,s^{-1}$ velocity range occupied by shocked H$\alpha$ emission. We examine the morphology and velocity structure of the HI and consider the origins of different components, finding some evidence that while the gas associated with NGC 7318B has been disturbed by its collision with the group, it may still retain a component of disk rotation. We find no gaseous connection between the tidal tails and NGC 7320C, but reaffirm the close connection between the shocked gas in the ridge (traced by X-ray, radio continuum and warm H$_2$ emission) and the southern tidal tail. Based on the integrated spectrum, we find a total HI mass in the group of 3.5$\pm$0.4$\times$10$^{10}\,M_{\odot}$, higher than the VLA estimate and comparable to FAST.

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arXiv2025

Magnetic Fields in the Shapley Supercluster Core with POSSUM: Challenging Model Predictions

Faraday Rotation Measure (RM) Grids provide a sensitive means to trace magnetized plasma across a wide range of cosmic environments. We study the RM signal from the Shapley Supercluster Core (SSC), in order to constrain the magnetic field properties of the gas. The SSC region consists of two galaxy clusters A3558 and A3562, and two galaxy groups between them, at $z\simeq 0.048$. We combine RM Grid data with thermal Sunyaev-Zeldovich effect data, obtained from the POSSUM pilot survey, and Planck, respectively. To robustly determine the gas density, its magnetic field properties, and their correlation, we study the RM scatter in the SSC region and its behavior as a function of distance to the nearest cluster/group. We compare observational results with semi-analytic Gaussian random field models and more realistic cosmological MHD simulations. With a sky-density of 36 RMs/deg$^{2}$, we detect an excess RM scatter of $30.5\pm 4.6 \, \mathrm{rad/m^2}$ in the SSC region. Comparing with models, we find an average magnetic field strength of 1-3 $μ$G (in the groups and clusters). The RM scatter profile, derived from data ranging from 0.3-1.8 $r_{500}$ for all objects, is systematically flatter than expected compared to models, with $η<0.5$ being favored. Despite this discrepancy, we find that cosmological MHD simulations matched to the SSC structure most closely align with scenarios where the magnetic field is amplified by the turbulent velocity in the intercluster regions on scales $\lesssim 0.8\,r_{500}$. The dense RM grid and precision provided by POSSUM allows us to probe magnetized gas in the SSC clusters and groups on scales within and beyond their $r_{500}$. Flatter-than-expected RM scatter profiles reveal a significant challenge in reconciling observations with even the most realistic predictions from cosmological MHD simulations in the outskirts of interacting clusters.

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arXiv2025

The Peculiar Mini-Halo in the Shapley Supercluster Member Abell 3558

We present a multi-band study of the diffuse emission in the galaxy cluster Abell 3558, located in the core of the Shapley Supercluster. Using new MeerKAT UHF-Band and uGMRT Band-3 observations, and published MeerKAT L-band and ASKAP 887 MHz data, we perform a detailed analysis of the diffuse emission in the cluster centre. We complement with XMM-Newton X-ray information for a thorough study of the connection between the thermal and non-thermal properties of the cluster. We find that the diffuse radio emission in the cluster centre is more extended than published earlier, with a previously undetected extension spanning 100 kpc towards the north beyond the innermost cold front, increasing the total size of the emission to 550 kpc, and shows a clear spatial correlation with the X-ray features. The overall radio spectrum is steep ($α_{\rm 400\,MHz}^{\rm 1569\,MHz}=1.18\pm0.10$), with local fluctuations which show several connections with the X-ray surface brightness, cold fronts, and residual emission. The point-to-point correlation between the radio and X-ray surface brightness is sub-linear, and steepens with increasing frequency. We discuss the classification of the diffuse emission considering its overall properties, those of the ICM, and the existing scaling laws between the radio and X-ray quantities in galaxy clusters. We conclude that it is a mini-halo, powered by turbulent (re)-acceleration induced by sloshing motions within the cluster region delimited by the cold fronts, and it supports the picture of a known minor merger between A\,3558 and the group SC\,1327--312 with mass ratio 5:1.

astro-ph.GA↗

arXiv2025

Relighting the fire in Hickson Compact Group (HCG) 15: magnetised fossil plasma revealed by the SKA Pathfinders & Precursors

In the context of the life cycle and evolution of active galactic nuclei (AGN), the environment plays an important role. In particular, the over-dense environments of galaxy groups, where dynamical interactions and bulk motions have significant impact, offer an excellent but under-explored window into the life cycles of AGN and the processes that shape the evolution of relativistic plasma. Pilot Survey observations with the Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) survey recovered diffuse emission associated with the nearby (z = 0.0228) galaxy group HCG15, which was revealed to be strongly linearly polarised. We study the properties of this emission in unprecedented detail to settle open questions about its nature and its relation to the group-member galaxies. We perform a multi-frequency spectropolarimetric study of HCG15 incorporating our ASKAP EMU observations as well as new data from MeerKAT, LOFAR, the GMRT, and the Karl G. Jansky Very Large Array (VLA), plus X-ray data from XMM-Newton and optical spectra from the Himalayan Chandra Telescope (HCT). Our study confirms that the diffuse structure represents remnant emission from historic AGN activity, likely associated with HCG15-D, some 80-86 Myr ago (based on ageing analysis). We detect significant highly linearly-polarised emission from a diffuse 'ridge'-like structure with a highly ordered magnetic field. Our analysis suggests that this emission is generated by draping of magnetic field lines in the intra-group medium (IGrM), although further exploration with simulations would aid our understanding. We confirm that HCG15-C is a group-member galaxy. Finally, we report the detection of thermal emission associated with a background cluster at redshift z ~ 0.87 projected onto the IGrM of HCG15, which matches the position and redshift of the recent SZ detection of ACT-CL J0207.8+0209.

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arXiv2025

A long-period radio transient active for three decades

Recently several long-period radio transients have been discovered, with strongly polarised coherent radio pulses appearing on timescales between tens to thousands of seconds [1,2]. In some cases the radio pulses have been interpreted as coming from rotating neutron stars with extremely strong magnetic fields, known as magnetars; the origin of other, occasionally periodic and less well-sampled radio transients, is still debated [3]. Coherent periodic radio emission is usually explained by rotating dipolar magnetic fields and pair production mechanisms, but such models do not easily predict radio emission from such slowly-rotating neutron stars and maintain it for extended times. On the other hand, highly magnetic isolated white dwarfs would be expected to have long spin periodicities, but periodic coherent radio emission has not yet been directly detected from these sources. Here we report observations of a long-period (21 minutes) radio transient, which we have labeled GPMJ1839-10. The pulses vary in brightness by two orders of magnitude, last between 30 and 300 seconds, and have quasi-periodic substructure. The observations prompted a search of radio archives, and we found that the source has been repeating since at least 1988. The archival data enabled constraint of the period derivative to $<3.6\times10^{-13}$s s$^{-1}$, which is at the very limit of any classical theoretical model that predicts dipolar radio emission from an isolated neutron star.

astro-ph.HE↗

arXiv2025

The head-tail radio galaxy and revived fossil plasma in Abell 1775

Head-tail radio galaxies are characterized by a head, corresponding to an elliptical galaxy, and two radio jets sweeping back from the head, forming an extended structure behind the host galaxy that is moving through the intracluster medium (ICM). This morphology arises from the interaction between the diffuse radio-emitting plasma and the surrounding environment. Sometimes revived fossil plasma is found in galaxy clusters, tracing old active galactic nucleus ejecta with a very steep spectrum re-energized through processes in the ICM, unrelated to the progenitor galaxy. We aim to study the central region of Abell 1775, a galaxy cluster in an unclear dynamical state at z = 0.072. It hosts two giant radio-loud elliptical galaxies, the head-tail radio galaxy that "breaks" at the position of a cold front detected in the X-rays, filamentary revived fossil plasma, and central diffuse emission. This study aims to investigate and constrain the spectral properties and trends along the head-tail, as well as the revived fossil plasma, to better understand the formation process of the non-thermal phenomena in A1775. We make use of LOFAR (144 MHz), and new deep uGMRT observations (400 and 650 MHz). We observe an overall steepening along the tail of the head-tail radio galaxy. In the radio colour-colour diagram, ageing models reproduce the emission of the head-tail. An unexpected brightness increase at the head of the tail suggests a complex bending of the jets. We derived the equipartition magnetic field and minimum pressure along the tail. We recovered the structure of the revived fossil plasma, which appears as thin filaments with ultra-steep spectra. We show that high-sensitivity, high-resolution observations at low frequencies are essential for detecting the full extent of the tail, enabling a deeper spectral analysis and resolving the structure and spectral properties of revived fossil plasma.

astro-ph.CO↗

arXiv2024

PSZ2 G282.28+49.94, a recently discovered analogue of the famous Bullet Cluster

We present a detailed study of the gas and galaxy properties of the cluster PSZ2 G282.28+49.94 detected in the Planck all-sky survey. The intracluster medium (ICM) of this object at z=0.56 exhibits a cometary-like shape. Combining Chandra and TNG observations, we characterised the spatially resolved thermodynamical properties of the gas and the spatial and velocity distribution of 73 galaxy members. The cluster structure is quite complex with an elongated core region containing the two brightest cluster galaxies and one dense group to the south-east. Since there is no velocity difference between the core and the south-east group, we suggest the presence of a merger along the plane of the sky. This structure is related to complex X-ray and radio features, and thus the merger has likely been caught during the post-merger phase. Comparing the distribution of the ICM and of member galaxies, we find a large offset of $\sim 350$ kpc between the position of the X-ray peak and the centre of a concentration of galaxies, preceding it in the likely direction of motion. This configuration is similar to the famous Bullet Cluster, leading us to dub PSZ2 G282.28+49.94 the "Planck bullet", and represents an ideal situation to provide astrophysical constraints to the self-interaction cross-section ($σ/m$) of dark matter particles. These results illustrate the power of a multi-wavelength approach to probe the merging scenario of such complex and distant systems.

astro-ph.CO↗

arXiv2024

A Deep Dive into the NGC 741 Galaxy Group: Insights into a Spectacular Head-Tail Radio Galaxy from VLA, MeerKAT, uGMRT and LOFAR

We present deep, wideband multifrequency radio observations (144 MHz$-$8 GHz) of the remarkable galaxy group NGC 741, which yield crucial insights into the interaction between the infalling head-tail radio galaxy (NGC 742) and the main group. Our new data provide an unprecedentedly detailed view of the NGC 741-742 system, including the shock cone, disrupted jets from NGC 742, the long ($\sim$ 255 kpc) braided southern radio tail, and eastern lobe-like structure, and reveal, for the first time, complex radio filaments throughout the tail and lobe, and a likely vortex ring behind the shock cone. The cone traces the bow shock caused by the supersonic ($\mathcal{M}\sim2$) interaction between the head-tail radio galaxy NGC 742 and the intragroup medium (IGrM) while the ring may have been formed by interaction between the NGC 742 shock and a previously existing lobe associated with NGC 741. This interaction plausibly compressed and re-accelerated the radio plasma. We estimate that shock-heating by NGC 742 has likely injected $\sim$2-5$\times$10$^{57}$ erg of thermal energy into the central 10 kpc cooling region of the IGrM, potentially affecting the cooling and feedback cycle of NGC 741. A comparison with Chandra X-ray images shows that some of the previously detected thermal filaments align with radio edges, suggesting compression of the IGrM as the relativistic plasma of the NGC 742 tail interacts with the surrounding medium. Our results highlight that multi-frequency observations are key to disentangling the complex, intertwined origins of the variety of radio features seen in the galaxy group NGC 741, and the need for simulations to reproduce all the detected features.

astro-ph.GA↗

arXiv2024

The prototypical major cluster merger Abell 754. I. Calibration of MeerKAT data and radio/X-ray spectral mapping of the cluster

Abell 754 is a rich galaxy cluster at $z=0.0543$ and is considered the prototype of a major cluster merger. Like many dynamically unrelaxed systems, it hosts diffuse radio emission on Mpc-scales. Extended synchrotron sources in the intra-cluster medium (ICM) are commonly interpreted as evidence that a fraction of the gravitational energy released during cluster mergers is dissipated into nonthermal components. Here, we use new MeerKAT UHF- and L-band observations to study nonthermal phenomena in Abell 754. These data are complemented with archival XMM-Newton observations to investigate the resolved spectral properties of both the radio and X-ray cluster emission.For the first time, we employed the pipeline originally developed to calibrate LOFAR data to MeerKAT observations. This allowed us to perform a direction-dependent calibration and obtain highly sensitive radio images in UHF- and L-bands which capture the extended emission with unprecedented detail. By using a large XMM-Newton mosaic, we produced thermodynamic maps of the ICM. Our analysis reveals that the radio halo in the cluster center is bounded by the well-known shock in the eastern direction. Furthermore, in the southwest periphery, we discover an extended radio source that we classify as a radio relic which is possibly tracing a shock driven by the squeezed gas compressed by the merger, outflowing in perpendicular directions. The low-luminosity of this relic appears compatible with direct acceleration of thermal pool electrons. We interpret the observed radio and X-ray features in the context of a major cluster merger with a nonzero impact parameter. Abell 754 is a remarkable galaxy cluster showcasing exceptional features associated with the ongoing merger event. The high quality of the new MeerKAT data motivates further work on this system.

astro-ph.CO↗

arXiv2024

Cosmic dance in the Shapley Concentration Core II. The uGMRT-MeerKAT view of filaments in the brightest cluster galaxies and tailed radio galaxies in the A3528 cluster complex

Superclusters are the largest-scale environments where a number of galaxy clusters interact with each other through minor/major mergers and grow via accretion along cosmic filaments. We focus on the A3528 complex in the core of the Shapley Supercluster. This system includes three clusters, A3528 (composed itself by two sub-clusters, namely A3528N and A3528S), A3532 and A3530, and presents a mildly active dynamical state. We study how minor mergers affect the evolution of radio galaxies and whether they are able to re-accelerate relativistic electrons in the ICM. We used observations from the uGMRT (Band 3, 4 and 5) and MeerKAT (L-band) telescopes to obtain images and spectral index maps over a wide frequency band and spatial resolutions. We compare these data with those from the SRG/eROSITA X-ray telescope. We detect faint diffuse radio emission associated with the radio galaxies. The BCGs in A3528S and A3532 show filaments of diffuse radio emission which extend for $\sim200-400$ kpc out of the radio galaxy. The spectral index of these filaments is extremely steep and almost constant ($α\sim -2, -2.5$). Contrary to the radio tails in A3528N, the spectral properties of these radio filaments are not consistent with standard models of plasma ageing. We also detect roundish diffuse radio emission around the BCG in A3528S which could be classified as a radio mini-halo. The radio tail in this cluster appears longer that in earlier detections, being $\sim300$ kpc long at all frequencies. We linked the presence of extended radio emission in the form of filaments and threads in the A3528 complex with the effect of minor mergers. This is reinforced by the increasing X-ray fluctuations in correspondence with the radio extended emission in A3528S. Despite the less energy involved, our findings support the hypothesis that these events can re-energise plasma originating from radio galaxies.

astro-ph.CO↗

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

An emission state switching radio transient with a 54 minute period

Long-period radio transients are an emerging class of extreme astrophysical events of which only three are known. These objects emit highly polarised, coherent pulses of typically a few tens of seconds duration and minutes to hour-long periods. While magnetic white dwarfs and magnetars, either isolated or in binary systems, have been invoked to explain these objects, a consensus has not emerged. Here we report on the discovery of ASKAP J193505.1+214841.0 (henceforth ASKAPJ1935+2148) with a period of 53.8 minutes exhibiting three distinct emission states - a bright pulse state with highly linearly polarised pulses with widths of 10-50 seconds; a weak pulse state which is about 26 times fainter than the bright state with highly circularly polarised pulses of widths of approximately 370 milliseconds; and a quiescent or quenched state with no pulses. The first two states have been observed to progressively evolve over the course of 8 months with the quenched state interspersed between them suggesting physical changes in the region producing the emission. A constraint on the radius of the source for the observed period rules out a magnetic white dwarf origin. Unlike other long-period sources, ASKAPJ1935+2148 is the first to exhibit drastic variations in emission modes reminiscent of neutron stars. However, its radio properties challenge our current understanding of neutron star emission and evolution.

astro-ph.HE↗

arXiv2023

Mining Mini-Halos with MeerKAT I. Calibration and Imaging

Radio mini-halos are clouds of diffuse, low surface brightness synchrotron emission that surround the Brightest Cluster Galaxy (BCG) in massive cool-core galaxy clusters. In this paper, we use third generation calibration (3GC), also called direction-dependent (DD) calibration, and point source subtraction on MeerKAT extragalactic continuum data. We calibrate and image archival MeerKAT L-band observations of a sample of five galaxy clusters (ACO 1413, ACO 1795, ACO 3444, MACS J1115.8+0129, MACS J2140.2-2339). We use the CARACal pipeline for direction-independent (DI) calibration, DDFacet and killMS for 3GC, followed by visibility-plane point source subtraction to image the underlying mini-halo without bias from any embedded sources. Our 3GC process shows a drastic improvement in artefact removal, to the extent that the local noise around severely affected sources was halved and ultimately resulted in a 7\% improvement in global image noise. Thereafter, using these spectrally deconvolved Stokes I continuum images, we directly measure for four mini-halos the flux density, radio power, size and in-band integrated spectra. Further to that, we show the in-band spectral index maps of the mini-halo (with point sources). We present a new mini-halo detection hosted by MACS J2140.2-2339, having flux density $S_{\rm 1.28\,GHz} = 2.61 \pm 0.31$ mJy, average diameter 296 kpc and $α^{\rm 1.5\,GHz}_{\rm 1\,GHz} = 1.21 \pm 0.36$. We also found a $\sim$100 kpc southern extension to the ACO 3444 mini-halo which was not detected in previous VLA L-band observations. Our description of MeerKAT wide-field, wide-band data reduction will be instructive for conducting further mini-halo science.

astro-ph.GA↗

arXiv2022

The detection of cluster magnetic fields via radio source depolarisation

It has been well established that galaxy clusters have magnetic fields. The exact properties and origin of these magnetic fields are still uncertain even though these fields play a key role in many astrophysical processes. Various attempts have been made to derive the magnetic field strength and structure of nearby galaxy clusters using Faraday rotation of extended cluster radio sources. This approach needs to make various assumptions that could be circumvented when using background radio sources. However, because the number of polarised radio sources behind clusters is low, at the moment such a study can only be done statistically. In this paper, we investigate the depolarisation of radio sources inside and behind clusters in a sample of 124 massive clusters at $z<0.35$ observed with the Karl G. Jansky Very Large Array. We detect a clear depolarisation trend with the cluster impact parameter, with sources at smaller projected distances to the cluster centre showing more depolarisation. By combining the radio observations with ancillary X-ray data from Chandra, we compare the observed depolarisation with expectations from cluster magnetic field models using individual cluster density profiles. The best-fitting models have a central magnetic field strength of $5-10\,μ$G with power-law indices between $n=1$ and $n=4$. We find no strong difference in the depolarisation trend between sources embedded in clusters and background sources located at similar projected radii, although the central region of clusters is still poorly probed by background sources. We also examine the depolarisation trend as a function of cluster properties such as the dynamical state, mass, and redshift. Our findings show that the statistical depolarisation of radio sources is a good probe of cluster magnetic field parameters. [abridged]

astro-ph.CO↗

arXiv2022

A candle in the wind: a radio filament in the core of the A3562 galaxy cluster

Using a MeerKAT observation of the galaxy cluster A3562 (a member of the Shapley Supercluster), we have discovered a narrow, long and straight, very faint radio filament, which branches out at a straight angle from the tail of a radio galaxy located in projection near the core of the cluster. The radio filament spans 200 kpc and aligns with a sloshing cold front seen in the X-rays, staying inside the front in projection. The radio spectral index along the filament appears uniform (within large uncertainties) at $α\simeq -1.5$. We propose that the radio galaxy is located outside the cold front, but dips its tail under the front. The tangential wind that blows there may stretch the radio plasma from the radio galaxy into a filamentary structure. Some reacceleration is needed in this scenario to maintain the radio spectrum uniform. Alternatively, the cosmic ray electrons from that spot in the tail can spread along the cluster magnetic field lines, straightened by that same tangential flow, via anomalously fast diffusion. Our radio filament can provide constraints on this process. We also uncover a compact radio source at the Brightest Cluster Galaxy (BCG) that is 2--3 orders of magnitude less luminous than those in typical cluster central galaxies -- probably an example of a BCG starved of accretion fuel by gas sloshing.

astro-ph.CO↗