Aims. We report on the discovery of four new radio galaxies with tailed morphology. Tailed radio galaxies are generally found in rich environments, therefore their presence can be used as tracer of a ...cluster. Methods. The radio galaxies were found in the fields of Giant Metrewave Radio Telescope (GMRT) observations carried out at 610 MHz and 327 MHz devoted to other studies. We inspected the literature and archives in the optical and X-ray bands to search for galaxy clusters or groups hosting them. Results. All the tailed radio galaxies serendipitously found in the GMRT fields are located in rich environments. Two of them belong to the candidate cluster NCS J090232+204358, located at $z_{\mathrm{phot}}$ = 0.0746; one belongs to the cluster MaxBCGJ 223.97317+22.15620 at $z_{\mathrm{phot}}$ = 0.2619; finally we suggest that the fourth one is probing a galaxy cluster at $z = 0.1177$, located behind A 262, and so far undetected in any band. Our results strengthen the relevance of high sensitivity and high resolution radio data in the detection of galaxy clusters at intermediate redshift.
The connection between the thermal and non-thermal properties in galaxy clusters hosting radio halos seems fairly well established. However, a comprehensive analysis of such a connection has only ...been done for integrated quantities (e.g. L X − P radio relation). In recent years, new-generation radio telescopes have enabled the unprecedented possibility to study the non-thermal properties of galaxy clusters on a spatially resolved basis. In this work, we performed a pilot study to investigate the mentioned properties on five targets by combining X-ray data from the CHEX-MATE project with the second data release from the LOFAR Two meter Sky survey. We find a strong correlation ( r s ∼ 0.7) with a slope less than unity between the radio and X-ray surface brightness. We also report differences in the spatially resolved properties of the radio emission in clusters that show different levels of dynamical disturbance. In particular, less perturbed clusters (according to X-ray parameters) show peaked radio profiles in the centre, with a flattening in the outer regions, while the three dynamically disturbed clusters have steeper profiles in the outer regions. We fitted a model to the radio emission in the context of turbulent re-acceleration with a constant ratio between thermal and non-thermal particles’ energy densities and a magnetic field profile linked to the thermal gas density as B ( r ) ∝ n th 0.5 . We found that this simple model cannot reproduce the behaviour of the observed radio emission.
We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high γ-ray activity state in 2012 March by ...the Large Area Telescope on board Fermi, becoming the γ-ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic γ-ray luminosity of 6.6 × 1049 erg s−1 which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift, Very Long Baseline Array, European VLBI Network, and Medicina single-dish telescope. Significant variability was observed from radio to X-rays supporting the identification of the γ-ray source with TXS 0536+145. Both the radio and γ-ray light curves show a similar behaviour, with the γ-rays leading the radio variability with a time lag of about 4-6 months. The luminosity increase is associated with a flattening of the radio spectrum. No new superluminal component associated with the flare was detected in high-resolution parsec-scale radio images. During the flare the γ-ray spectrum seems to deviate from a power law, showing a curvature that was not present during the average activity state. The γ-ray properties of TXS 0536+145 are consistent with those shown by the high-redshift γ-ray blazar population.
ABSTRACT
In this paper, we present results from a radio–optical study of the galaxy populations of the galaxy clusters Abell 1300 and MACS J1931.8−2634, a merger and a relaxed system respectively ...both located at z ∼ 0.3, aimed at finding evidence of merger-induced radio emission. Radio observations are taken at 1.28 GHz with the MeerKAT interferometer during its early-stage commissioning phase, and combined with archive optical data. We generated catalogues containing 107 and 162 radio sources in the A 1300 and MACS J1931.8−2634 cluster fields, respectively, above a 0.2 mJy threshold and within a 30 arcmin radius from the cluster centre (corresponding to 8.1 and 8.8 Mpc, respectively). By cross-correlating the radio and optical catalogues, and including spectroscopic information, nine and six sources were found to be cluster members and used to construct the radio luminosity functions, respectively, for both clusters. The comparison of the radio source catalogues between the two cluster fields leads to a marginal difference, with a 2σ statistical significance. We derived the radio luminosity function at 1.28 GHz in both clusters, in the power range of $22.81 \lt \rm {log~\mathit{ P}_{1.28~GHz}~(W\,Hz^{-1})} \lt 25.95$, and obtained that in A 1300 the radio luminosity function averaged over the full radio power interval is only 3.3 ± 1.9 times higher than the MACS J1931.8−2634 one, suggesting no statistical difference in their probability to host nuclear radio emission. We conclude that, at least for the two clusters studied here, the role of cluster mergers in affecting the statistical properties of the radio galaxy population is negligible.
Context. A number of merging galaxy clusters show the presence of large-scale radio emission associated with the intra-cluster medium (ICM). These synchrotron sources are generally classified as ...radio haloes and radio relics. Aims. Whilst it is commonly accepted that mergers play a crucial role in the formation of radio haloes and relics, not all the merging clusters show the presence of giant diffuse radio sources and this provides important information concerning current models. The Abell 781 complex is a spectacular system composed of an apparent chain of clusters on the sky. Its main component is undergoing a merger and hosts peripheral emission that is classified as a candidate radio relic and a disputed radio halo. Methods. We used new LOw Frequency ARay (LOFAR) observations at 143 MHz and archival Giant Metrewave Radio Telescope (GMRT) observations at 325 and 610 MHz to study radio emission from non-thermal components in the ICM of Abell 781. Complementary information came from XMM-Newton data, which allowed us to investigate the connection with the thermal emission and its complex morphology. Results. The origin of the peripheral emission is still uncertain. We speculate that it is related to the interaction between a head tail radio galaxy and shock. However, the current data allow us only to set an upper limit of ℳ < 1.4 on the Mach number of this putative shock. Instead, we successfully characterise the surface brightness and temperature jumps of a shock and two cold fronts in the main cluster component of Abell 781. Their positions suggest that the merger is involving three substructures. We do not find any evidence for a radio halo either at the centre of this system or in the other clusters of the chain. We place an upper limit to the diffuse radio emission in the main cluster of Abell 781 that is a factor of 2 below the current radio power-mass relation for giant radio haloes.
Aims. We present a detailed study of the giant radio halo in the galaxy cluster Abell 697 to constrain its origin and connection with the cluster dynamics. Methods. We performed high sensitivity GMRT ...observations at 325 MHz, which showed that the radio halo is much brighter and larger at this frequency than in previous 610 MHz observations. To derive the integrated spectrum in the frequency range 325 MHz-1.4 GHz, we reanalysed archival VLA data at 1.4 GHz and used proprietary GMRT data at 610 MHz. Results. Our multifrequency analysis shows that the total radio spectrum of the giant radio halo in A 697 is very steep, with $\alpha_{\rm~325\, MHz}^{\rm~1.4\, GHz}$ ≈ 1.7–1.8. Owing to energy arguments, a hadronic origin of the halo is disfavoured by this steep spectrum. Very steep spectrum halos in merging clusters are predicted in the case that the emitting electrons are accelerated by turbulence. Observations with upcoming low frequency arrays will be able to test these expectations.
Context. Thermal gas in the center of galaxy clusters can show substantial motions that generate surface-brightness and temperature discontinuities known as cold fronts. The motions may be triggered ...by minor or off-axis mergers that preserve the cool core of the system. The dynamics of the thermal gas can also generate radio emission from the intra-cluster medium (ICM) and impact the evolution of clusters’ radio sources. Aims. We aim to study the central region of Abell 1775, a system in an ambiguous dynamical state at z = 0.072 which is known to host an extended head-tail radio galaxy, with the goal of investigating the connection between thermal and nonthermal components in its center. Methods. We made use of a deep (100 ks) Chandra observation accompanied by LOFAR 144 MHz, GMRT 235 MHz and 610 MHz, and VLA 1.4 GHz radio data. Results. We find a spiral-like pattern in the X-ray surface brightness that is mirrored in the temperature and pseudo-entropy maps. Additionally, we characterize an arc-shaped cold front in the ICM. We interpret these features in the context of a slingshot gas tail scenario. The structure of the head-tail radio galaxy “breaks” at the position of the cold front, showing an extension that is detected only at low frequencies, likely due to its steep and curved spectrum. We speculate that particle reacceleration is occurring in the outer region of this tail, which in total covers a projected size of ∼800 kpc. We also report the discovery of revived fossil plasma with ultra-steep spectrum radio emission in the cluster core together with a central diffuse radio source that is bounded by the arc-shaped cold front. Conclusions. The results reported in this work demonstrate the interplay between thermal and nonthermal components in the cluster center and the presence of ongoing particle reacceleration in the ICM on different scales.
Aims. We investigate the possible presence of diffuse radio emission in the intermediate redshift, massive cluster PLCKG285.0-23.7 (z= 0.39, M sub(500)= 8.39 x 10 super(14)M sub(middot in circle)). ...Methods. Our 16 cm-band ATCA observations of PLCKG285.0-23.7 allow us to reach a rms noise level of ~11 mu Jy/beam on the wide-band (1.1-3.1 GHz), full-resolution (~5 arcsec) image of the cluster, making it one of the deepest ATCA images yet published. We also re-image visibilities at lower resolution in order to achieve a better sensitivity to low-surface-brightness extended radio sources. Results. We detect one of the lowest luminosity radio halos known at z> 0.35, characterised by a slight offset from the well-studied 1.4 GHz radio power vs. cluster mass correlation. Similarly to most known radio-loud clusters (i.e. those hosting diffuse non-thermal sources), PLCKG285.0-23.7 has a disturbed dynamical state. Our analysis reveals a similarly elongated X-ray and radio morphology. While the size of the radio halo in PLCKG285.0-23.7 is smaller than lower redshift radio-loud clusters in the same mass range, it shows a similar correlation with the cluster virial radius, as expected in the framework of hierarchical structure formation.
PDF
