Abstract
We present deep XMM-Newton, Karl G. Jansky Very Large Array, and upgraded Giant Metrewave Radio Telescope observations of Abell 746, a cluster that hosts a plethora of diffuse emission ...sources that provide evidence for the acceleration of relativistic particles. Our new XMM-Newton images reveal a complex morphology of the thermal gas with several substructures. We observe an asymmetric temperature distribution across the cluster: the southern regions exhibit higher temperatures, reaching ∼9 keV, while the northern regions have lower temperatures (≤4 keV), likely due to a complex merger. We find evidence of three surface brightness edges and one candidate edge, of which three are merger-driven shock fronts. Combining our new data with published LOw-Frequency ARray observations has unveiled the nature of diffuse sources in this system. The bright NW relic shows thin filaments and a high degree of polarization with aligned magnetic field vectors. We detect a density jump, aligned with the fainter relic to the north. To the south, we detect high-temperature regions, consistent with the shock-heated regions and a density jump coincident with the northern tip of the southern radio structure. Its integrated spectrum shows a high-frequency steepening. Lastly, we find that the cluster hosts large-scale radio halo emission. A comparison of the thermal and nonthermal emission reveals an anticorrelation between the bright radio and X-ray features at the center. Our findings suggest that Abell 746 is a complex system that involves multiple mergers.
ABSTRACT
Low-frequency radio observations show an increasing number of radio galaxies located in galaxy clusters that display peculiar morphologies and spectral profiles. This is the result of the ...dynamical interaction of the galaxy with the surrounding medium. Studying this phenomenon is key to understanding the evolution of low-energy relativistic particles in the intracluster medium. We present a multifrequency study of the three head–tail (HT) radio galaxies and the radio halo in the galaxy cluster ZwCl 0634.1+4747. We make use of observations at four frequencies performed with LOFAR LBA (53 MHz), HBA (144 MHz), GMRT (323 MHz), and VLA (1518 MHz) data. The use of extremely low radio frequency observations, such as LOFAR at 53 and 144 MHz, allowed us to detect the extension of the tails up to a distance of ∼1 Mpc. We extracted spectral profiles along the tails in order to identify possible departures from a pure ageing model, such as the Jaffe–Perola (JP) model, which only involves synchrotron and inverse-Compton losses. We found clear evidence of departures from this simple ageing model, such as surface brightness enhancement and spectral flattening along all of the tails. This can be interpreted as the consequence of particle re-acceleration along the tails. Possible explanations for this behaviour include the interaction between a shock and the radio tails or a turbulence-driven re-acceleration mechanism. We show that the latter scenario is able to reproduce the characteristic features that we observed in our profiles.
ABSTRACT
Many relaxed cool-core clusters host diffuse radio emission on scales of hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for generating them, as well as their ...connection with central active galactic nuclei, is elusive and many questions related to their physical properties and origins remain unanswered. This paper presents new radio observations of the galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a moderately disturbed non-cool-core cluster. Our study reveals an asymmetric mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported at similar frequencies. The spectral index is flatter than previously reported, with an integrated value of α = −1.01 ± 0.06, shows significant spatial variation, and a tentative radial steepening. We studied the point-to-point X-ray/radio surface brightness correlation to investigate the thermal/non-thermal connection: our results show a strong connection between these components, with a superlinear slope of $b = 1.63^{+0.10}_{-0.10}$ at 1283 MHz and $b = 1.20^{+0.13}_{-0.11}$ at 145 MHz. We also explore the X-ray surface brightness/radio spectral index correlation, finding a slope of $b = 0.59^{+0.11}_{-0.11}$. Both investigations support the evidence of spectral steepening. Finally, in the context of understanding the particle acceleration mechanism, we present a simple theoretical model which demonstrates that hybrid scenarios – secondary electrons (re-)accelerated by turbulence – reproduce a superlinear correlation slope.
Context.
The relation between giant radio halos and mini halos in galaxy clusters is not understood. The former are usually associated with merging clusters, while the latter are found in relaxed ...systems. In recent years, the advent of low-frequency radio observations has challenged this dichotomy by finding intermediate objects with a hybrid radio morphology.
Aims.
We aim to investigate the presence of diffuse radio emission in the cluster Abell 1413 and determine its dynamical status to explore the relation between mini halos and giant radio halos.
Methods.
We used LOFAR observations centred at 144 MHz to study the diffuse radio emission. To investigate the dynamical state of the system, we used newly analysed
XMM-Newton
archival data. Abell 1413 shows features that are typically present in both relaxed (e.g., peaked X-ray surface brightness distribution and some large-scale inhomogeneities) and disturbed (e.g., flatter temperature and metallicity profiles) clusters.
Results.
This suggests that Abell 1413 is neither disturbed nor fully relaxed, and we argue that it is an intermediate-phase cluster. At 144 MHz, we discover a wider diffuse component surrounding the previously known mini halo at the cluster center. By fitting the radio surface-brightness profile with a double-exponential model, we can disentangle the two components. We find an inner mini halo with an
e
-folding radius,
r
e
, 1
= 28 ± 5 kpc, and an extended component with
r
e
, 2
= 290 ± 60 kpc. We also evaluated the point-to-point correlation between the radio and X-ray surface brightness, finding a sublinear relation for the outer emission and a superlinear relation for the mini halo. The mini halo and the diffuse emission extend over different scales and show different features, confirming the double nature of the radio emission and suggesting that the mechanisms responsible for the re-acceleration of the radio-emitting particle might be different.
Context.
Turbulence introduced into the intracluster medium (ICM) through cluster-merger events transfers energy to non-thermal components, and can trigger the formation of diffuse synchrotron radio ...sources. Typical diffuse sources in the form of giant radio halos and mini-halos are found in merging and relaxed cool-core galaxy clusters, respectively. On the other hand, recent observations reveal an increasing complexity to the non-thermal phenomenology.
Aims.
Abell 2142 (A2142) is a mildly disturbed cluster that exhibits uncommon thermal and non-thermal properties. It is known to host a hybrid halo consisting of two components (H1 and H2), namely a mini-halo-like and an enigmatic elongated radio halo-like structure. We aim to investigate the properties, origin, and connections of each component.
Methods.
We present deep LOFAR observations of A2142 in the frequency ranges 30–78 MHz and 120 − 168 MHz. With complementary multi-frequency radio and X-ray data, we analysed the radio spectral properties of the halo and assessed the connection between the non-thermal and thermal components of the ICM.
Results.
We detect a third radio component (H3), which extends over the cluster volume on scales of ∼2 Mpc, embeds H1 and H2, and has a morphology that roughly follows the thermal ICM distribution. The radio spectral index is moderately steep in H1 (
α
= 1.09 ± 0.02) and H2 (
α
= 1.15 ± 0.02), but is steeper (
α
= 1.57 ± 0.20) in H3. Our analysis of the thermal and non-thermal properties allowed us to discuss possible formation scenarios for each radio component. Turbulence from sloshing motions of low-entropy gas on different scales may be responsible for the origin of H1 and H2. We classified H3 as a giant ultrasteep spectrum radio halo, and find that it may trace the residual activity from an old energetic merger and/or inefficient turbulent reacceleration induced by ongoing minor mergers.
Low-frequency radio observations show an increasing number of radio galaxies located in galaxy clusters that display peculiar morphologies and spectral profiles. This is the result of the dynamical ...interaction of the galaxy with the surrounding medium. Studying this phenomenon is key to understanding the evolution of low-energy relativistic particles in the intracluster medium. We present a multi-frequency study of the three head-tail (HT) radio galaxies and the radio halo in the galaxy cluster ZwCl0634.1+4747. We make use of observations at four frequencies performed with LOFAR LBA (53 MHz), HBA (144 MHz), GMRT (323 MHz) and VLA (1518 MHz) data. The use of extremely low radio frequency observations, such as LOFAR at 53 and 144 MHz, allowed us to detect the extension of the tails up to a distance of ~ 1 Mpc. We extracted spectral profiles along the tails in order to identify possible departures from a pure ageing model, such as the Jaffe-Perola (JP) model, which only involves synchrotron and inverse-Compton losses. We found clear evidence of departures from this simple ageing model, such as surface brightness enhancement and spectral flattening along all of the tails. This can be interpreted as the consequence of particle re-acceleration along the tails. Possible explanations for this behaviour include the interaction between a shock and the radio tails or a turbulence-driven re-acceleration mechanism. We show that the latter scenario is able to reproduce the characteristic features that we observed in our profiles.
Aims.
In this work, we analysed new LOw Frequency ARray observations of the mini halo in the cluster RBS 797, together with archival Very Large Array observations and the recent
Chandra
results. This ...cluster is known to host a powerful active galactic nucleus (AGN) at its centre, with two pairs of jets propagating in orthogonal directions. Recent X-ray observations have detected three pairs of shock fronts within 125 kpc from the cluster centre, connected with the activity of the central AGN. Our aim is to investigate the connection between the mini halo emission and the activity of the central source.
Methods.
We have used different methods to separate the emission of the central source from the diffuse mini halo emission, and we have derived the radial spectral index trend of the mini halo.
Results.
We find that the diffuse radio emission is elongated in different directions at 144 MHz (east-west) with respect to 1.4 GHz (north-south), tracing the orientation of the two pairs of jets. The mini halo emission is characterised by an average spectral index
α
= −1.02 ± 0.05. The spectral index profile of the mini halo shows a gradual flattening from the centre to the periphery. Such a trend is unique among the mini halos studied to date, and resembles the spectral index trend typical of particles re-accelerated by shocks. However, the estimated contribution to the radio brightness profile coming from shock re-acceleration is found to be insufficient to account for the radial brightness profile of the mini halo.
Conclusions.
We propose three scenarios that could explain the observed trend: (i) the AGN-driven shocks are propagating onto an already existing mini halo, re-energising the electrons and leaving clear imprints in the mini halo spectral properties. We estimate that the polarisation induced by the shocks could be detected at 6 GHz and above; (ii) we could be witnessing turbulent re-acceleration in a high magnetic field cluster; and (iii) the mini halo could have a hadronic origin, in which the particles are injected by the central AGN and the diffusion coefficient depends of the cosmic ray proton momentum. Future observations in polarisation would be fundamental to understand the role of shocks and of the magnetic field.
Many relaxed cool-core clusters host diffuse radio emission on scales of hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for generating them, as well as their connection with ...central active galactic nuclei, is elusive and many questions related to their physical properties and origins remain unanswered. This paper presents new radio observations of the galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a moderately-disturbed non-cool-core cluster. Our study reveals an asymmetric mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported at similar frequencies. The spectral index is flatter than previously reported, with an integrated value of $\alpha = -1.01 \pm 0.06$, shows significant spatial variation, and a tentative radial steepening. We studied the point-to-point X-ray/radio surface brightness correlation to investigate the thermal/non-thermal connection: our results show a strong connection between these components, with a super-linear slope of $b = 1.63 \pm 0.10$ at 1283 MHz and $b = 1.20 \pm 0.12$ at 145 MHz. We also explore the X-ray surface brightness/radio spectral index correlation, finding a slope of $b = 0.59 \pm 0.11$. Both investigations support the evidence of spectral steepening. Finally, in the context of understanding the particle acceleration mechanism, we present a simple theoretical model which demonstrates that hybrid scenarios - secondary electrons (re-)accelerated by turbulence - reproduce a super-linear correlation slope.
Low-frequency radio observations show an increasing number of radio galaxies
located in galaxy clusters that display peculiar morphologies and spectral
profiles. This is the result of the dynamical ...interaction of the galaxy with
the surrounding medium. Studying this phenomenon is key to understanding the
evolution of low-energy relativistic particles in the intracluster medium. We
present a multi-frequency study of the three head-tail (HT) radio galaxies and
the radio halo in the galaxy cluster ZwCl0634.1+4747. We make use of
observations at four frequencies performed with LOFAR LBA (53 MHz), HBA (144
MHz), GMRT (323 MHz) and VLA (1518 MHz) data. The use of extremely low radio
frequency observations, such as LOFAR at 53 and 144 MHz, allowed us to detect
the extension of the tails up to a distance of ~ 1 Mpc. We extracted spectral
profiles along the tails in order to identify possible departures from a pure
ageing model, such as the Jaffe-Perola (JP) model, which only involves
synchrotron and inverse-Compton losses. We found clear evidence of departures
from this simple ageing model, such as surface brightness enhancement and
spectral flattening along all of the tails. This can be interpreted as the
consequence of particle re-acceleration along the tails. Possible explanations
for this behaviour include the interaction between a shock and the radio tails
or a turbulence-driven re-acceleration mechanism. We show that the latter
scenario is able to reproduce the characteristic features that we observed in
our profiles.
In this work, we analysed new LOw Frequency ARray observations of the mini halo in the cluster RBS797, together with archival Very Large Array observations and the recent Chandra results. This ...cluster is known to host a powerful active galactic nucleus (AGN) at its centre, with two pairs of jets propagating in orthogonal directions. Recent X-ray observations have detected three pairs of shock fronts, connected with the activity of the central AGN. Our aim is to investigate the connection between the mini halo emission and the activity of the central source. We find that the diffuse radio emission is elongated in different directions at 144 MHz (east-west) with respect to 1.4 GHz (north-south), tracing the orientation of the two pairs of jets. The mini halo emission is characterised by an average spectral index \(\alpha=-1.02\pm 0.05\). The spectral index profile of the mini halo shows a gradual flattening from the centre to the periphery. Such a trend is unique among the mini halos studied to date, and resembles the spectral index trend typical of particles re-accelerated by shocks. However, the estimated contribution to the radio brightness profile coming from shock re-acceleration is found to be insufficient to account for the radial brightness profile of the mini halo. We propose three scenarios that could explain the observed trend: (i) the AGN-driven shocks are propagating onto an already existing mini halo, re-energising the electrons. We estimate that the polarisation induced by the shocks could be detected at 6 GHz and above; (ii) we could be witnessing turbulent re-acceleration in a high magnetic field cluster; and (iii) the mini halo could have a hadronic origin, in which the particles are injected by Future observations in polarisation would be fundamental to understand the role of shocks and the magnetic field.
