A. Bonafede

The Coma cluster magnetic field from Faraday rotation measures

Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical simulations of the magnetic field. Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B0 and radial power-law slope η ,w hereη indicates how the field scales with respect to the gas density profile. Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ∼2 kpc and maximum scale ∼34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B0 = 3.9 μG; η = 0.4) and (B0 = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B0 = 4.7 μG; η = 0.5. Values of B0 > 7 μ Ga nd 1.0 are incompatible with RM data at 99% confidence level.

Lensing and x-ray mass estimates of clusters (simulations)

We present a comparison between weak-lensing and x-ray mass estimates of a sample of numerically simulated clusters. The sample consists of the 20 most massive objects at redshift z = 0.25 and M_vir > 5 × 10^(14) M_☉ h^(−1). They were found in a cosmological simulation of volume 1 h^(−3) Gpc^3, evolved in the framework of a WMAP-7 normalized cosmology. Each cluster has been resimulated at higher resolution and with more complex gas physics. We processed it through Skylens and X-MAS to generate optical and x-ray mock observations along three orthogonal projections. The final sample consists of 60 cluster realizations. The optical simulations include lensing effects on background sources. Standard observational tools and methods of analysis are used to recover the mass profiles of each cluster projection from the mock catalogue. The resulting mass profiles from lensing and x-ray are individually compared to the input mass distributions. Given the size of our sample, we could also investigate the dependence of the results on cluster morphology, environment, temperature inhomogeneity and mass. We confirm previous results showing that lensing masses obtained from the fit of the cluster tangential shear profiles with Navarro–Frenk–White functionals are biased low by ~5–10% with a large scatter (~10–25%). We show that scatter could be reduced by optimally selecting clusters either having regular morphology or living in substructure-poor environment. The x-ray masses are biased low by a large amount (~25–35%), evidencing the presence of both non-thermal sources of pressure in the intra-cluster medium (ICM) and temperature inhomogeneity, but they show a significantly lower scatter than weak-lensing-derived masses. The x-ray mass bias grows from the inner to the outer regions of the clusters. We find that both biases are weakly correlated with the third-order power ratio, while a stronger correlation exists with the centroid shift. Finally, the x-ray bias is strongly connected with temperature inhomogeneities. Comparison with a previous analysis of simulations leads to the conclusion that the values of x-ray mass bias from simulations are still uncertain, showing dependences on the ICM physical treatment and, possibly, on the hydrodynamical scheme adopted.

Radio halos in nearby (z < 0.4) clusters of galaxies

Context. The intra-cluster medium is characterized by thermal emission, and by the presence of large scale magnetic fields. In some clusters of galaxies, a diffuse non-thermal emission is also present, located at the cluster center and called radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume. Aims. In this paper we collect data on all known nearby cluster radio halos (z < 0.4), to discuss their statistical properties and to investigate their origin. Methods. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos. Results. We present data on 31 radio halos, 1 new relic source, and 1 giant filament. We note the discovery of a small size diffuse radio emission in a cluster (A1213) with very low X-ray luminosity. Among the statistical results, we confirm the correlation between the average halo radio spectral index and the cluster temperature. We also discuss the high percentage of clusters where both a relic and a radio halo is present. Conclusions. The sample of radio halos discussed here represents the population of radio halos observable with current radio telescopes. A new telescope generation is necessary for a more detailed multifrequency study, and to investigate the possible existence of a population of radio halos with different properties.

An improved map of the galactic Faraday sky

We aim to summarize the current state of knowledge regarding Galactic Faraday rotation in an all-sky map of the Galactic Faraday depth. For this we have assembled the most extensive catalog of Faraday rotation data of compact extragalactic polarized radio sources to date. In the map-making procedure we used a recently developed algorithm that reconstructs the map and the power spectrum of a statistically isotropic and homogeneous field while taking into account uncertainties in the noise statistics. This procedure is able to identify some rotation angles that are offset by an integer multiple of π. The resulting map can be seen as an improved version of earlier such maps and is made publicly available, along with a map of its uncertainty. For the angular power spectrum we find a power law behavior C� ∝ � −2.17 for a Faraday sky where an overall variance profile as a function of Galactic latitude has been removed, in agreement with earlier work. We show that this is in accordance with a 3D Fourier power spectrum P(k) ∝ k −2.17 of the underlying

Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745

Aims. To study at multiple frequencies the radio emission arising from the massive galaxy cluster MACS J0717.5+3745 (z = 0.55). Known to be an extremely complex cluster merger, the system is uniquely suited for an investigation of the phenomena at work in the intra-cluster medium (ICM) during cluster collisions. Methods. We use multi-frequency and multi-resolution data obtained with the Very Large Array radio telescope, and X-ray features revealed by Chandra, to probe the non-thermal and thermal components of the ICM, their relations and interactions. Results. The cluster shows highly complex radio emission. A bright, giant radio halo is detected at frequencies as high as 4.8 GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a radio halo. This radio halo is also the most powerful ever observed, and the second case for which polarized radio emission has been detected, indicating that the magnetic field is ordered on large scales.

Double relics in Abell 2345 and Abell 1240 - Spectral index and polarization analysis

Aims. The aim of the present work is to study the radio properties of double relics in Abell 1240 and Abell 2345 in the framework of double relic formation models. Methods. We present new Very Large Array observations at 20 and 90 cm for these two clusters. We performed spectral index and polarization analysis and compared our results with expectations from theoretical models. Results. The presence of double relics in these two cluster is confirmed by these new observations. Double relics in Abell 1240 show radio morphology, spectral index, and polarization values in agreement with “outgoing merger shocks” models. One of the relics of Abell 2345 shows a peculiar morphology and spectral index profile, which are difficult to reconcile with present scenarios. We suggest a possible origin for this peculiar relic.

Discovery of radio haloes and double relics in distant MACS galaxy clusters: clues to the efficiency of particle acceleration

We have performed 323 MHz observations with the Giant Metrewave Radio Telescope of the most promising candidates selected from the MACS catalogue. The aim of the work is to extend our knowledge of the radio halo and relic populations to z > 0.3, the epoch in which massive clusters are formed. In MACSJ1149.5+2223 and MACSJ1752.1+4440, we discovered two double-relic systems with a radio halo, and in MACSJ0553.4−3342 we found a radio halo. Archival Very Large Array observations and Westerbork Synthesis Radio Telescope observations have been used to study the polarization and spectral-index properties. The radio halo in MACSJ1149.5+2223 has the steepest spectrum ever found so far in these objects (α ≥ 2). The double relics in MACSJ1149.5+2223 are peculiar in their position that is misaligned with the main merger axis. The relics are polarized up to 30 and 40 per cent in MACSJ1149.5+2223 and MACSJ1752.040+44, respectively. In both cases, the magnetic field is roughly aligned with the relics’ main axes. The spectra in the relics in MACSJ1752.040+44 steepen towards the cluster centre, in agreement with model expectations. X-ray data on MACSJ0553.4−3342 suggest that this cluster is undergoing a major merger, with the merger axis close to the plane of the sky. The cores of the disrupted clusters have just passed each other, but no radio relic is detected in this system. If turbulence is responsible for the radio emission, we argue that it must develop before the core passage. A comparison of double-relic plus halo system with cosmological simulations allows a simultaneous estimate of the acceleration efficiencies at shocks (to produce relics) and of turbulence (to produce the halo).

Probing the origin of giant radio haloes through radio and γ-ray data: the case of the Coma cluster

We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-Large Area Telescope (LAT) and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons. First, we investigate the case of pure secondary models that are merely based on the mechanism of continuous injection of secondary electrons via proton–proton collisions in the intracluster medium. We use the observed spatial distribution of the halos radio brightness to constrain the amount of cosmic ray protons and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of cosmic rays is a power law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of cosmic ray protons necessary to explain the spectrum of the halo and the very broad spatial distribution (and large energy density) of cosmic rays result in a γ-ray emission in excess of present limits, unless the cluster magnetic field is relatively large. However, this large magnetic field required to not violate present γ-ray limits appears inconsistent with that derived from recent Faraday rotation measures. Secondly, we investigate more complex models in which the cosmic rays confined diffusively in the Coma cluster and their secondary electrons are all reaccelerated by magnetohydrodynamics turbulence. We show that under these conditions it is possible to explain the radio spectrum and morphology of the radio halo and to predict γ-ray fluxes in agreement with the Fermi-LAT upper limits without tension with present constraints on the cluster magnetic field. Reacceleration of secondary particles also requires a very broad cosmic ray spatial profile, much flatter than that of the intracluster medium, at least provided that both the turbulent and magnetic field energy densities scale with that of the intracluster medium. However, this requirement can be easily alleviated if we assume that a small amount of (additional) seed primary electrons is reaccelerated in the clusters external regions, or if we adopt flatter scalings of the turbulent and magnetic field energy densities with distance from the cluster centre.

Rotation measures of radio sources in hot galaxy clusters

Aims. The goal of this work is to investigate the Faraday rotation measure (RM) of radio galaxies in hot galaxy clusters in order to establish a possible connection between the magnetic field strength and the gas temperature of the intracluster medium. Methods. We performed Very Large Array observations at 3.6 cm and 6 cm of two radio galaxies located in A401 and Ophiuchus, a radio galaxy in A2142, and a radio galaxy located in the background of A2065. All these galaxy clusters are characterized by high temperatures. Results. We obtained detailed RM images at an angular resolution of 3 �� for most of the observed radio galaxies. The RM images are patchy and reveal fine substructures of a few kpc in size. Under the assumption that the radio galaxies themselves have no effect on the measured RMs, these structures indicate that the intracluster magnetic fields fluctuate down to such small scales. These new data are compared with RM information present in the literature for cooler galaxy clusters. For a fixed projected distance from the cluster center, clusters with higher temperature show a higher dispersion of the RM distributions (σRM), mostly because of the higher gas density in these clusters. Although the previously known relation between the clusters X-ray surface brightness (S X) at the radio galaxy location and σRM is confirmed, a possible connection between the σRM − S X relation and the cluster temperature, if present, is very weak. Therefore, in view of the current data, it is impossible to establish a strict link between the magnetic field strength and the gas temperature of the intracluster medium.

Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes

Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.