Many galaxy clusters host Megaparsec-scale radio halos, generated by ultrarelativistic electrons in the magnetized intracluster medium. Correlations between the synchrotron power of radio halos and the thermal properties of the hosting clusters were established in the last decade, including the connection between the presence of a halo and cluster mergers. The X-ray luminosity and redshift limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for statistical studies of the halos. We uniformly analyze the radio and X-ray data for the GMRT cluster sample, and use the new Planck SZ catalog, to revisit the correlations between the power of radio halos and the thermal properties of galaxy clusters. We find that the radio power at 1.4 GHz scales with the cluster X-ray (0.1-2.4 keV) luminosity computed within R 500 as P 1.4 ∼ L 2.1±0.2 500. Our bigger and more homogenous sample confirms that the X-ray luminous (L 500 > 5 × 10 44 erg s −1 ) clusters branch into two populations -radio halos lie on the correlation, while clusters without radio halos have their radio upper limits well below that correlation. This bimodality remains if we excise cool cores from the X-ray luminosities. We also find that P 1.4 scales with the cluster integrated SZ signal within R 500 , measured by Planck, as P 1.4 ∼ Y 2.05±0.28 500 , in line with previous findings. However, contrary to previous studies that were limited by incompleteness and small sample size, we find that "SZluminous" Y 500 > 6 × 10 −5 Mpc 2 clusters show a bimodal behavior for the presence of radio halos, similar to that in the radio-X-ray diagram. Bimodality of both correlations can be traced to clusters dynamics, with radio halos found exclusively in merging clusters. These results confirm the key role of mergers for the origin of giant radio halos, suggesting that they trigger the relativistic particle acceleration.

The frequently observed association between giant radio halos and merging galaxy clusters has driven present theoretical models of non-thermal emission from galaxy clusters, which are based on the idea that the energy dissipated during cluster-cluster mergers could power the formation of radio halos. To quantitatively test the merger-halo connection we present the first statistical study based on deep radio data and X-ray observations of a complete X-ray selected sample of galaxy clusters with X-ray luminosity ≥ 5 × 10 44 erg/s and redshift 0.2 ≤ z ≤ 0.32. Using several methods to characterize cluster substructures, namely the power ratios, centroid shift and X-ray brightness concentration parameter, we show that clusters with and without radio halo can be quantitatively differentiated in terms of their dynamical properties. In particular, we confirm that radio halos are associated to dynamically disturbed clusters and cluster without radio halo are more "relaxed", with only a couple of exceptions where a disturbed cluster does not exhibit a halo.

Aims. We present the first results of an ongoing project devoted to the search of giant radio halos in galaxy clusters located in the redshift range z=0.2-0.4. One of the main goals of our study is to measure the fraction of massive galaxy clusters in this redshift interval hosting a radio halo, and to constrain the expectations of the particle re-acceleration model for the origin of non-thermal radio emission in galaxy clusters. Methods. We selected 27 REFLEX clusters and here we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for 11 of them. The sensitivity (1σ) in our images is in the range 35-100 µJy beam −1 for all clusters. Results. We found three new radio halos, doubling the number of halos known in the selected sample. In particular, giant radio halos were found in A 209 and RXCJ 2003.5-2323, and one halo (of smaller size) was found in RXCJ 1314.4-2515. Candidate extended emission on smaller scale was found around the central galaxy in A 3444 which deserves further investigation. Furthermore, a radio relic was found in A 521, and two relics were found in RXCJ 1314.5-2515. The remaining six clusters observed do not host extended emission of any kind.

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.Comment: 18 pages, 4 figures, Nature 455, 94

There is now firm evidence that the intracluster medium (ICM) consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidence for non‐thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scales observed in a growing number of massive clusters and, more recently, in the hard X‐ray tails detected in a few cases in excess of the thermal bremsstrahlung spectrum. A promising possibility to explain giant radio haloes is given by the presence of relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events. With the aim of investigating the connection between thermal and non‐thermal properties of the ICM, in this paper we develop a statistical magneto‐turbulent model which describes in a self‐consistent way the evolution of the thermal ICM and that of the non‐thermal emission from clusters. Making use of the extended Press–Schechter formalism, we follow cluster mergers and estimate the injection rate of the fluid turbulence generated during these energetic events. We then calculate the evolution of the spectrum of the relativistic electrons in the ICM during the cluster life by taking into account both the electron acceleration due to the merger‐driven turbulence and the relevant energy losses of the electrons. We end up with a synthetic population of galaxy clusters for which the evolution of the ICM and of the non‐thermal spectrum emitted by the accelerated electrons is calculated. The generation of detectable non‐thermal radio and hard X‐ray emission in the simulated clusters is found to be possible during major merger events for reliable values of the model parameters. In addition the occurrence of radio haloes as a function of the mass of the parent clusters is calculated and compared with observations. In this case it is found that the model expectations are in good agreement with observations: radio haloes are found in about 30 per cent of the more massive clusters in our synthetic population (M≳ 1.8 × 1015 M⊙) and in about 4 per cent of the intermediate massive clusters (9 × 1014 < M < 1.8 × 1015 M⊙), while the radio halo phenomenon is found to be extremely rare in the case of the smaller clusters.

Aims. Giant radio halos are diffuse, Mpc-scale, synchrotron sources located in the central regions of galaxy clusters and provide the most relevant example of cluster non-thermal activity. Radio and X-ray surveys allow to investigate the statistics of radio halos and may contribute to constrain the origin of these sources and their evolution. Methods. We investigate the distribution of clusters in the plane X-ray (thermal, L X ) vs. synchrotron (non-thermal, P 1.4 ) luminosity, where clusters hosting giant radio halos trace the P 1.4 -L X correlation and clusters without radio halos populate a region that is well separated from that spanned by the above correlation. The connection between radio halos and cluster mergers suggests that the cluster Mpc-scale synchrotron emission is amplified during these mergers and then suppressed when clusters become more dynamically relaxed. Results. In this context, by analysing the distribution in the P 1.4 -L X plane of galaxy clusters from X-ray selected samples with adequate radio follow up, we constrain the typical time-scale of evolution of diffuse radio emission in clusters and discuss the implications for the origin of radio halos. Conclusions. We conclude that cluster synchrotron emission is suppressed (and amplified) in a time-scale significantly smaller than 1 Gyr. We show that this constraint appears difficult to reconcile with the hypothesis that the halo's radio power is suppressed due to dissipation of magnetic field in galaxy clusters. On the other hand, in agreement with models where turbulent acceleration plays a role, present constraints suggest that relativistic electrons are accelerated in Mpc-scale regions, in connection with cluster mergers and for a time-interval of about 1 Gyr, and then they cool in a relatively small time-scale, when the hosting cluster becomes more dynamically relaxed.

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 per cent of the turbulence in clusters is in the form of compressible modes.In this work, we extend the above-mentioned analysis by including a scaling of the magnetic field strength with cluster mass. We show that, in the framework of the reacceleration model, the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B ∝ M b v , with b 0.5 and typical μG strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained: the evolution of such a probability depends on the interplay between synchrotron and inverse Compton losses in the emitting volume, and it is maximized in clusters for which the two losses are comparable.The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P 1.4 GHz ∼ 10 24 W Hz −1 , and severely cut off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters, and that the occurrence of GRHs at 1.4 GHz beyond a redshift z ∼ 0.7 appears to be negligible. As a related check, we also show that the predicted integral radio source counts within a limited volume (z 0.2) are consistent with present observational constraints. Extending the source counts beyond z = 0.2, we estimate that the total number of GRHs to be discovered at ∼ mJy radio fluxes could be ∼100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the forthcoming operation of low-frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam−1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.