Turbulent Re-acceleration Research Articles

The discovery of radio halos in the frontier fields clusters Abell S1063 and Abell 370

Context. Massive merging galaxy clusters often host diffuse megaparsec-scale radio synchrotron emission. This emission originates from relativistic electrons in the ionized intracluster medium. An important question is how these synchrotron emitting relativistic electrons are accelerated. Aims. Our aim is to search for diffuse emission in the Frontier Fields clusters Abell S1063 and Abell 370 and characterize its properties. While these clusters are very massive and well studied at some other wavelengths, no diffuse emission has been reported for these clusters so far. Methods. We obtained 325 MHz Giant Metrewave Radio Telescope (GMRT) and 1–4 GHz Jansky Very Large Array (VLA) observations of Abell S1063 and Abell 370. We complement these data with Chandra and XMM-Newton X-ray observations. Results. In our sensitive images, we discover radio halos in both clusters. In Abell S1063, a giant radio halo is found with a size of ∼1.2 Mpc. The integrated spectral index between 325 MHz and 1.5 GHz is −0.94 ± 0.08 and it steepens to −1.77 ± 0.20 between 1.5 and 3.0 GHz. This spectral steepening provides support for the turbulent reacceleration model for radio halo formation. Abell 370 hosts a faint radio halo mostly centered on the southern part of this binary merging cluster, with a size of ∼500−700 kpc. The spectral index between 325 MHz and 1.5 GHz is −1.10 ± 0.09. Both radio halos follow the known scaling relation between the cluster mass proxy Y500 and radio power, which is consistent with the idea that they are related to ongoing cluster merger events.

ASKAP reveals giant radio halos in two merging SPT galaxy clusters

AbstractEarly science observations from the Australian Square Kilometre Array Pathfinder (ASKAP) have revealed clear signals of diffuse radio emission associated with two clusters detected by the South Pole Telescope via their Sunyaev Zel’dovich signal: SPT CLJ0553-3342 (MACS J0553.4-3342) and SPT CLJ0638-5358 (Abell S0592) are both high-mass lensing clusters that have undergone major mergers. To create science-fidelity images of the galaxy clusters, we performed direction-dependent (DD) calibration and imaging on these ASKAP early science observations using state-of-the-art software killMS and DDFacet. Here, we present our DD calibrated ASKAP radio images of both clusters showing unambiguous giant radio halos with largest linear scales of${\sim}1$Mpc. The halo in MACS J0553.4-3342 was previously detected with Giant Metrewave Radio Telescope observations at 323 MHz but appears more extended in our ASKAP image. Although there is a shock detected in the thermal X-ray emission of this cluster, we find that the particle number density in the shocked region is too low to allow for the generation of a radio shock. The radio halo in Abell S0592 is a new discovery, and the Southwest border of the halo coincides with a shock detected in X-rays. We discuss the origins of these halos considering both the hadronic and turbulent re-acceleration models and sources ofseedelectrons. This work gives a positive indication of the potential of ASKAP’s Evolutionary Map of the Universe survey in detecting intracluster medium radio sources.

Contribution of Radio Halos to the Foreground for SKA EoR Experiments

The overwhelming foreground contamination is one of the primary impediments to probing the Epoch of Reionization (EoR) through measuring the redshifted 21 cm signal. Among various foreground components, radio halos are less studied and their impacts on the EoR observations are still poorly understood. In this work, we employ the Press-Schechter formalism, merger-induced turbulent re-acceleration model, and the latest SKA1-Low layout configuration to simulate the SKA "observed" images of radio halos. We calculate the one-dimensional power spectra from simulated images and find that radio halos can be about $10^4$, $10^3$ and $10^{2.5}$ times more luminous than the EoR signal on scales of $0.1\,\text{Mpc}^{-1} < k < 2\,\text{Mpc}^{-1}$ in the 120-128, 154-162, and 192-200 MHz bands, respectively. By examining the two-dimensional power spectra inside properly defined EoR windows, we find that the power leaked by radio halos can still be significant, as the power ratios of radio halos to the EoR signal on scales of $0.5\,\text{Mpc}^{-1} \lesssim k \lesssim 1\,\text{Mpc}^{-1}$ can be up to about 230-800%, 18-95%, and 7-40% in the three bands, when the 68% uncertainties caused by the variation of the number density of bright radio halos are considered. Furthermore, we find that radio halos located inside the far side-lobes of the station beam can also impose strong contamination within the EoR window. In conclusion, we argue that radio halos are severe foreground sources and need serious treatments in future EoR experiments.

Effect of turbulent reacceleration on electrons produced by dark matter annihilation in the Coma cluster

ABSTRACT In this paper, we study the effect of reacceleration provided by turbulences on electrons produced by dark matter (DM) annihilation in the Coma cluster. We use a simplified phenomenological model to describe the effect of the turbulences, and explore a limited subset of three possible DM models for neutralino particles with different mass and annihilation channel. We find that, for values of the annihilation cross-section of the order of the upper limits found with Fermi–LAT measurements in astrophysical objects, and for conservative values of the boosting factor due to DM substructures, the reacceleration due to turbulences can enhance the radio emission produced by DM-originated electrons up to the level of the observed flux of the radio halo in Coma, for moderate reacceleration intensity in relatively short times. Therefore, we conclude that, even if it is not possible to distinguish between the fits obtained in this paper because of the scattering present in the radio flux data, the electrons produced by DM annihilation can be possible seed electrons for the reacceleration, as well as secondary electrons of hadronic origin. A possible discriminant between these two classes of models is the flux produced in the gamma ray band, which in the case of DM-originated electrons should be more than two orders of magnitude smaller than the present Fermi–LAT upper limits, whereas in the hadronic case the expected gamma ray flux should be close to the value of present upper limits.

Evolutionary phases of merging clusters as seen by LOFAR

Massive, merging galaxy clusters often host giant, diffuse radio sources that arise from shocks and turbulence; hence, radio observations can be useful for determining the merger state of a cluster. In preparation for a larger study, we selected three clusters – Abell 1319, Abell 1314, and RXC J1501.3+4220 (Z7215) – making use of the new LOFAR Two-Metre Sky Survey (LoTSS) at 120–168 MHz, and together with archival data, show that these clusters appear to be in pre-merging, merging, and post-merging states, respectively. We argue that Abell 1319 is likely in its pre-merging phase, where three separate cluster components are about to merge. There are no radio haloes nor radio relics detected in this system. Abell 1314 is a highly disturbed, low-mass cluster that is likely in the process of merging. This low-mass system does not show a radio halo, however, we argue that the merger activates mechanisms that cause electron re-acceleration in the large 800 kpc radio tail associated with IC 711. In the cluster Z7215 we discover diffuse radio emission at the cluster centre, and we classify this emission as a radio halo, although it is dimmer and smaller than expected by the radio halo power versus cluster mass correlation. We suggest that the disturbed cluster Z7215 is in its post-merging phase. Systematic studies of this kind over a larger sample of clusters observed with LoTSS will help to constrain the timescales involved in turbulent re-acceleration and the subsequent energy losses of the underlying electrons.

On the Connection between Turbulent Motions and Particle Acceleration in Galaxy Clusters

Abstract Giant radio halos are megaparsec-scale diffuse radio sources associated with the central regions of galaxy clusters. The most promising scenario to explain the origin of these sources is that of turbulent re-acceleration, in which MeV electrons injected throughout the formation history of galaxy clusters are accelerated to higher energies by turbulent motions mostly induced by cluster mergers. In this Letter, we use the amplitude of density fluctuations in the intracluster medium as a proxy for the turbulent velocity and apply this technique to a sample of 51 clusters with available radio data. Our results indicate a segregation in the turbulent velocity of radio halo and radio quiet clusters, with the turbulent velocity of the former being on average higher by about a factor of two. The velocity dispersion recovered with this technique correlates with the measured radio power through the relation , which implies that the radio power is nearly proportional to the turbulent energy rate. In case turbulence cascades without being dissipated down to the particle acceleration scales, our results provide an observational confirmation of a key prediction of the turbulent re-acceleration model and possibly shed light on the origin of radio halos.

On the absence of radio haloes in clusters with double relics

Pairs of radio relics are believed to form during cluster mergers, and are best observed when the merger occurs in the plane of the sky. Mergers can also produce radio halos, through complex processes likely linked to turbulent re-acceleration of cosmic-ray electrons. However, only some clusters with double relics also show a radio halo. Here, we present a novel method to derive upper limits on the radio halo emission, and analyse archival X-ray Chandra data, as well as galaxy velocity dispersions and lensing data, in order to understand the key parameter that switches on radio halo emission. We place upper limits on the halo power below the $P_{\rm 1.4 \, GHz}\, M_{500}$ correlation for some clusters, confirming that clusters with double relics have different radio properties. Computing X-ray morphological indicators, we find that clusters with double relics are associated with the most disturbed clusters. We also investigate the role of different mass-ratios and time-since-merger. Data do not indicate that the merger mass ratio has an impact on the presence or absence of radio halos (the null hypothesis that the clusters belong to the same group cannot be rejected). However, the data suggests that the absence of radio halos could be associated with early and late mergers, but the sample is too small to perform a statistical test. Our study is limited by the small number of clusters with double relics. Future surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to better address this issue.

Clusters of Galaxies and the Cosmic Web with Square Kilometre Array

The intra-cluster and inter-galactic media (ICM, IGM) that pervade the large scale structure of the Universe are known to be magnetised at sub-micro Gauss to micro Gauss levels and to contain cosmic rays (CRs). The acceleration of CRs and their evolution along with that of magnetic fields in these media is still not well understood. Diffuse radio sources of synchrotron origin associated with the ICM such as radio halos, relics and mini-halos are direct probes of the underlying mechanisms of CR acceleration. Observations with radiotelescopes such as the GMRT, the VLA and the WSRT (0.15 - 2 GHz) have revealed scaling relations between the thermal and non-thermal properties of clusters and favour the role of shocks in the formation of radio relics and of turbulent re-acceleration in the formation of radio halos and mini-halos. Due to the limitations of current radio telescopes, wide-band studies and exploration of low mass and supercluster-scale systems is difficult. The Square Kilometer Array (SKA) is a next generation radio telescope that will operate in the frequency range of 0.05 - 20 GHz with unprecedented sensitivities and resolutions. The expected detection limits of SKA will reveal a few hundred to thousand new radio halos, relics and mini-halos providing the first large and comprehensive samples for their study. The wide frequency coverage along with sensitivity to extended structures will be able to constrain the CR acceleration mechanisms. The higher frequency (> 5 GHz) observations will be able to use the Sunyaev-Zel'dovich effect to probe the ICM pressure in addition to the tracers such as lobes of head-tail radio sources. The SKA also opens prospects to detect the "off-state" radio emission from the ICM predicted by the hadronic models and the turbulent re-acceleration models. [abridged]

First detection at 5.5 and 9 GHz of the radio relics in bullet cluster with ATCA

We present here results from observations at 5.5 and 9 GHz of the Bullet cluster 1E 0657-55.8 with the Australia Telescope Compact Array (ATCA). Our results show detection of diffuse emission in the cluster. Our findings are consistent with the previous observations by Shimwell et al. (2014) and Shimwell et al. (2015) at 1.1-3.1 GHz. Morphology of diffuse structures (relic regions A and B and the radio halo) are consistent with those reported by the previous study. Our results indicate steepening in the spectral index at higher frequencies (at and greater than 5.0 GHz) for region A. The spectrum can be fit well by a broken power law. We discuss the possibility of a few recent theoretical models explaining this break in the power law spectrum, and find that a modified Diffusive Shock Acceleration (DSA) model or a turbulent reacceleration model may be relevant. Deep radio observations at high frequencies (at and greater than 5 GHz) are required for a detailed comparison with this model.

Stochastic reacceleration of relativistic electrons by turbulent reconnection: a mechanism for cluster-scale radio emission?

In this paper we investigate a situation where relativistic particles are reaccelerated diffusing across regions of reconnection and magnetic dynamo in super-Alfvenic, incompressible large-scale turbulence. We present an exploratory study of this mechanism in the intra-cluster-medium (ICM). In view of large-scale turbulence in the ICM we adopt a reconnection scheme that is based on turbulent reconnection and MHD turbulence. In this case particles are accelerated and decelerated in a systematic way in reconnecting and magnetic-dynamo regions, respectively, and on longer time-scales undergo a stochastic process diffusing across these sites (similar to second-order Fermi). Our study extends on larger scales numerical studies that focused on the acceleration in and around turbulent reconnecting regions. We suggest that this mechanism may play a role in the reacceleration of relativistic electrons in galaxy clusters providing a new physical scenario to explain the origin of cluster-scale diffuse radio emission. Indeed differently from current turbulent reacceleration models proposed for example for radio halos this mechanism is based on the effect of large-scale incompressible and super-Alfvenic turbulence. In this new model turbulence governs the interaction between relativistic particles and magnetic field lines that diffuse, reconnect and are stretched in the turbulent ICM.

TURBULENT COSMIC-RAY REACCELERATION AT RADIO RELICS AND HALOS IN CLUSTERS OF GALAXIES

Radio relics are synchrotron emission found on the periphery of galaxy clusters. From the position and the morphology, it is often believed that the relics are generated by cosmic-ray (CR) electrons accelerated at shocks through a diffusive shock acceleration (DSA) mechanism. However, some radio relics have harder spectra than the prediction of the standard DSA model. One example is observed in the cluster 1RXS J0603.3+4214, which is often called the ``Toothbrush Cluster''. Interestingly, the position of the relic is shifted from that of a possible shock. In this study, we show that these discrepancies in the spectrum and the position can be solved if turbulent (re)acceleration is very effective behind the shock. This means that for some relics turbulent reacceleration may be the main mechanism to produce high-energy electrons, contrary to the common belief that it is the DSA. Moreover, we show that for efficient reacceleration, the effective mean free path of the electrons has to be much smaller than their Coulomb mean free path. We also study the merging cluster 1E 0657-56 or the ``Bullet Cluster'', in which a radio relic has not been found at the position of the prominent shock ahead of the bullet. We indicate that a possible relic at the shock is obscured by the observed large radio halo that is generated by strong turbulence behind the shock. We propose a simple explanation if the morphological differences of radio emission among the Toothbrush, the Bullet, and the Sausage (CIZA J2242.8+5301) Clusters.

MAPPING THE PARTICLE ACCELERATION IN THE COOL CORE OF THE GALAXY CLUSTER RX J1720.1+2638

We present new deep, high-resolution radio images of the diffuse minihalo in the cool core of the galaxy cluster RX J1720.1+2638. The images have been obtained with the Giant Metrewave Radio Telescope at 317, 617 and 1280 MHz and with the Very Large Array at 1.5, 4.9 and 8.4 GHz, with angular resolutions ranging from 1" to 10". This represents the best radio spectral and imaging dataset for any minihalo. Most of the radio flux of the minihalo arises from a bright central component with a maximum radius of ~80 kpc. A fainter tail of emission extends out from the central component to form a spiral-shaped structure with a length of ~230 kpc, seen at frequencies 1.5 GHz and below. We find indication of a possible steepening of the total radio spectrum of the minihalo at high frequencies. Furthermore, a spectral index image shows that the spectrum of the diffuse emission steepens with the increasing distance along the tail. A striking spatial correlation is observed between the minihalo emission and two cold fronts visible in the Chandra X-ray image of this cool core. These cold fronts confine the minihalo, as also seen in numerical simulations of minihalo formation by sloshing-induced turbulence. All these observations favor the hypothesis that the radio emitting electrons in cluster cool cores are produced by turbulent reacceleration.

THE RADIO RELICS AND HALO OF EL GORDO, A MASSIVEz= 0.870 CLUSTER MERGER

We present 610 MHz and 2.1 GHz imaging of the massive SZE-selected z=0.870 cluster merger ACT-CL J0102-4915 (El Gordo), obtained with the GMRT and the ATCA, respectively. We detect two complexes of radio relics separated by 3.4' (1.6 Mpc) along the system's NW-to-SE collision axis that have high integrated polarizations (33%) and steep spectral indices, consistent with creation via Fermi acceleration by shocks in the ICM. From the spectral index of the relics, we compute a Mach number of 2.5^{+0.7}_{-0.3} and shock speed of 2500^{+400}_{-300} km/s. With our ATCA data, we compute the Faraday depth across the NW relic and find a mean value of 11 rad/m^2 and standard deviation of 6 rad/m^2. With the integrated line-of-sight gas density derived from new Chandra observations, our Faraday depth measurement implies B_parallel~0.01 \mu G in the cluster outskirts. The extremely narrow shock widths in the relics (<23 kpc) prevent us from placing a meaningful constraint on |B| using cooling time arguments. In addition to the relics, we detect a large (1.1 Mpc radius), powerful (log L_1.4[W/Hz]= 25.66+-0.12) radio halo with a Bullet-like morphology. The spectral-index map of the halo shows the synchrotron spectrum is flattest near the relics, along the collision axis, and in regions of high T_gas, all locations associated with recent energy injection. The spatial and spectral correlation between the halo emission and cluster X-ray properties supports primary-electron processes like turbulent reacceleration as the halo production mechanism. The halo's integrated 610 MHz to 2.1 GHz spectral index is 1.2+-0.1, consistent with the cluster's high T_gas in view of previously established global scaling relations. El Gordo is the highest-redshift cluster known to host a radio halo and/or radio relics, and provides new constraints on the non-thermal physics in clusters at z>0.6. [abridged]

153 MHz GMRT follow-up of steep-spectrum diffuse emission in galaxy clusters

In this paper we present new high sensitivity 153 MHz Giant Meterwave Radio Telescope follow-up observations of the diffuse steep spectrum cluster radio sources in the galaxy clusters Abell 521, Abell 697, Abell 1682. Abell 521 hosts a relic, and together with Abell 697 it also hosts a giant very steep spectrum radio halo. Abell 1682 is a more complex system with candidate steep spectrum diffuse emission. We imaged the diffuse radio emission in these clusters at 153 MHz, and provided flux density measurements of all the sources at this frequency. Our new flux density measurements, coupled with the existing data at higher frequencies, allow us to study the total spectrum of the halos and relic over at least one order of magnitude in frequency. Our images confirm the presence of a very steep "diffuse component" in Abell 1682. We found that the spectrum of the relic in Abell 521 can be fitted by a single power-law with $\alpha=1.45\pm0.02$ from 153 MHz to 5 GHz. Moreover, we confirm that the halos in Abell 521 and Abell 697 have a very steep spectrum, with $\alpha=1.8-1.9$ and $\alpha=1.52\pm0.05$ respectively. Even with the inclusion of the 153 MHz flux density information it is impossible to discriminate between power-law and curved spectra, as derived from homogeneous turbulent re-acceleration. The latter are favored on the basis of simple energetic arguments, and we expect that LOFAR will finally unveil the shape of the spectra of radio halos below 100 MHz, thus providing clues on their origin.

Detection of a radio bridge in Abell 3667

We have detected a radio bridge of unpolarized synchrotron emission connecting the NW relic of the galaxy cluster Abell 3667 to its central regions. We used data at 2.3 GHz from the S-band Polarization All Sky Survey (S-PASS) and at 3.3 GHz from a follow up observation, both conducted with the Parkes Radio Telescope. This emission is further aligned with a diffuse X-ray tail, and represents the most compelling evidence for an association between intracluster medium turbulence and diffuse synchrotron emission. This is the first clear detection of a bridge associated both with an outlying cluster relic and X-ray diffuse emission. All the indicators point toward the synchrotron bridge being related to the post-shock turbulent wake trailing the shock front generated by a major merger in a massive cluster. Although predicted by simulations, this is the first time such emission is detected with high significance and clearly associated with the path of a confirmed shock. Although the origin of the relativistic electrons is still unknown, the turbulent re-acceleration model provides a natural explanation for the large-scale emission. The equipartition magnetic field intensity of the bridge is B_eq = 2.2 +/- 0.3 \mu G. We further detect diffuse emission coincident with the central regions of the cluster for the first time.

Radio halos in future surveys in the radio continuum

Giant radio halos (RH) are Mpc-scale synchrotron sources detected in a significant fraction of massive and merging galaxy clusters.Their statistical properties can be used to discriminate among various models for their origin. Theoretical predictions are important as new radio telescopes are about to begin to survey the sky at low and high frequencies with unprecedented sensitivity. We carry out Monte Carlo simulations to model the formation and evolution of RH in a cosmological framework by assuming that RH are either generated in turbulent merging clusters, or are purely hadronic sources generated in more relaxed clusters, "off-state" halos. The models predict that the luminosity function of RH at high radio luminosities is dominated by the contribution of RH generated in turbulent clusters. The generation of these RH becomes less efficient in less massive systems causing a flattening of the luminosity function at lower luminosities. This flattening is compensated by the contribution of "off-state" RH that dominate at lower luminosities. By restricting to clusters at z<0.6, we show that the planned EMU+WODAN surveys at 1.4 GHz have the potential to detect up to ~200 RH, increasing their number by one order of magnitude. A fraction of these sources will be "off-state" RH that should be found at flux level < 10 mJy, presently accessible only to deep pointed observations. We also explore the synergy between the Tier 1 LOFAR survey at 150 MHz and the EMU+WODAN surveys at 1.4 GHz. We predict a larger number of RH in the LOFAR survey due to the high LOFAR sensitivity, but also due to the existence of RH with very steep spectrum that glow up preferentially at lower frequencies. These RH are only predicted in the framework of turbulent re-acceleration models and should not have counterparts in the EMU+WODAN surveys, thus the combination of the two surveys will test theoretical models.

Structure and turbulence in simulated galaxy clusters and the implications for the formation of radio haloes

We track the histories of massive clusters of galaxies formed within a cosmological hydrodynamic simulation. Specifically, we track the time evolution of the energy in random bulk motions of the intracluster medium and X-ray measures of cluster structure and their relationship to cluster mergers. We aim to assess the viability of the turbulent re-acceleration model for the generation of giant radio halos by comparing the level of turbulent kinetic energy in simulated clusters with the observed properties of radio halo clusters, giving particular attention to the association of radio halos to clusters with disturbedX-ray structures. The evolution of X-ray cluster structure and turbulence kinetic energy, k, in simulations can then inform us about the expected lifetime of radio halos and the fraction of clusters as a function of redshift expected to host them. We find strong statistical correlation of disturbed structure measures and the presence of enhancements in k. Specifically, quantitatively "disturbed", radio halo-like X-ray morphology in our sample indicates a 92% chance of the cluster in question having k elevated to more than twice its minimum value over the cluster's life. The typical lifetime of episodes of elevated turbulence is on the order of 1 Gyr, though these periods can last 5 Gyrs or more. This variation reflects the wide range of cluster histories; while some clusters undergo complex and repeated mergers spending a majority of their time in elevated k states, other clusters are relaxed over nearly their entire history. We do not find a bimodal relationship between cluster X-ray luminosity and the total energy in turbulence that might account directly for a bimodal L_X-P_{1.4 GHz} relation. However, our result may be consistent with the observed bimodality, as here we are not including a full treatment of cosmic rays sources and magnetic fields.

An unlikely radio halo in the low X-ray luminosity galaxy cluster RXC J1514.9-1523

We report the discovery of a giant radio halo in the galaxy cluster RXC J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, $L_{X \, [0.1-2.4 \rm \, kev]} \sim 7 \times 10^{44}$ erg s$^{-1}$. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. The integrated radio spectrum of the halo is quite steep, with a slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with $L_X \ltsim 8 \times 10^{44}$ erg s$^{-1}$. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the recently discovered population of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration.

Diffuse radio emission in/around the Coma cluster: beyond simple accretion

We report on new 1.41 GHz Green Bank Telescope and 352 MHz Westerbork Synthesis Radio Telescope observations of the Coma cluster and its environs. At 1.41 GHz we tentatively detect an extension to the Coma cluster radio relic source 1253+275 which makes its total extent ~2 Mpc. This extended relic is linearly polarized as seen in our GBT data, the NVSS, and archival images, strengthening a shock interpretation. The extended relic borders a previously undetected "wall" of galaxies in the infall region of the Coma cluster. We suggest that the radio relic is an infall shock, as opposed to the outgoing merger shocks believed responsible for other radio relics. We also find a sharp edge, or "front", on the western side of the 352 MHz radio halo. This front is coincident with a similar discontinuity in the X-ray surface brightness and temperature in its southern half, suggesting a primary shock-acceleration origin for the local synchrotron emitting electrons. The northern half of the synchrotron front is less well correlated with the X-ray properties, perhaps due to projection effects. We confirm the global pixel-to-pixel power-law correlation between the 352 MHz radio brightness and X-ray brightness with a slope that is inconsistent with predictions of either primary shock acceleration or secondary production of relativistic electrons in Giant Radio Halos, but is allowable in the framework of the turbulent re-acceleration of relic plasma. The failure of these first order models and the need for a more comprehensive view of the intracluster medium energization is also highlighted by the very different shapes of the diffuse radio and X-ray emission. We note the puzzling correspondence between the shape of the brighter regions of the radio halo and the surface mass density derived from weak lensing.

EVOLUTION OF SHOCKS AND TURBULENCE IN MAJOR CLUSTER MERGERS

We performed a set of cosmological simulations of major mergers in galaxy clusters to study the evolution of merger shocks and the subsequent injection of turbulence in the post-shock region and in the intra-cluster medium (ICM). The computations were done with the grid-based, adaptive mesh refinement hydro code Enzo, using an especially designed refinement criteria for refining turbulent flows in the vicinity of shocks. A substantial amount of turbulence energy is injected in the ICM due to major merger. Our simulations show that the shock launched after a major merger develops an ellipsoidal shape and gets broken by the interaction with the filamentary cosmic web around the merging cluster. The size of the post-shock region along the direction of shock propagation is about 300 kpc h^-1, and the turbulent velocity dispersion in this region is larger than 100 km s^-1. Scaling analysis of the turbulence energy with the cluster mass within our cluster sample is consistent with M^(5/3), i.e. the scaling law for the thermal energy in the self-similar cluster model. This clearly indicates the close relation between virialization and injection of turbulence in the cluster evolution. We found that the ratio of the turbulent to total pressure in the cluster core within 2 Gyr after the major merger is larger than 10%, and it takes about 4 Gyr to get relaxed, which is substantially longer than typically assumed in the turbulent re-acceleration models, invoked to explain the statistics of observed radio halos. Striking similarities in the morphology and other physical parameters between our simulations and the "symmetrical radio relics" found at the periphery of the merging cluster A3376 are finally discussed. In particular, the interaction between the merger shock and the filaments surrounding the cluster could explain the presence of "notch-like" features at the edges of the double relics.