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A. Bonafede

Publications and source records attributed to A. Bonafede.

At least 199 records · Page 11Linked to original sources
arXiv2012

Diffuse radio emission in MACS J1752.0+4440

We report the discovery of large-scale diffuse radio emission in the galaxy cluster MACS J1752.0+4440 (z=0.366). Using Westerbork Synthesis Radio Telescope (WSRT) observations we find that the cluster hosts a double radio relic system as well as a 1.65 Mpc radio halo covering the region between the two relics. The relics are diametrically located on opposite sides of the cluster center. The NE and SW relics have sizes of 1.3 and 0.9 Mpc, respectively. In case of an isolated binary merger event, the relative sizes of the relics suggest a mass ratio about 2:1. We measure integrated spectra of -1.16 \pm 0.03 for the NE and -1.10 \pm 0.05 for the SW relic. We conclude that this cluster has undergone a violent binary merger event and the relics are best explained by particles (re)accelerated in outwards traveling shock waves. The spectral indices suggest the relics trace shock waves with Mach numbers (M) of around 3.5 to 4.5. These relatively high Mach numbers derived from the radio spectral index are comparable to those derived for a few other recently discovered relics. This implies that merger shocks with M > 3 are relatively common in cluster outskirts if our understanding of diffusive shock acceleration is correct.

astro-ph.CO↗
arXiv2012

Wide-band Simultaneous Observations of Pulsars: Disentangling Dispersion Measure and Profile Variations

Dispersion in the interstellar medium is a well known phenomenon that follows a simple relationship, which has been used to predict the time delay of dispersed radio pulses since the late 1960s. We performed wide-band simultaneous observations of four pulsars with LOFAR (at 40-190 MHz), the 76-m Lovell Telescope (at 1400 MHz) and the Effelsberg 100-m Telescope (at 8000 MHz) to test the accuracy of the dispersion law over a broad frequency range. In this paper we present the results of these observations which show that the dispersion law is accurate to better than 1 part in 100000 across our observing band. We use this fact to constrain some of the properties of the ISM along the line-of-sight and use the lack of any aberration or retardation effects to determine upper limits on emission heights in the pulsar magnetosphere. We also discuss the effect of pulse profile evolution on our observations, and the implications that it could have for precision pulsar timing projects such as the detection of gravitational waves with pulsar timing arrays.

astro-ph.HE↗
arXiv2012

First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR Low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of -0.81 \pm 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last \sim 0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.

astro-ph.CO↗
arXiv2012

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 use 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 pi. 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 with a power law index of -2.14 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) proportional to k^-2.14 of the underlying field n_e times B_r under simplifying geometrical and statistical assumptions.

astro-ph.GA↗
arXiv2012

Lensing and X-ray mass estimates of clusters (SIMULATION)

[Abridged] We present a comparison between weak-lensing (WL) and X-ray mass estimates of a sample of numerically simulated clusters. The sample consists on the 20 most massive objects at redshift z=0.25 and Mvir > 5 x 10^{14} Msun 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 thought Skylens and X-MAS to generate optical and X-ray mock observations along three orthogonal projections. 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 catalogues. 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 WL masses obtained from the fit of the cluster tangential shear profiles with NFW 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 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.

astro-ph.CO↗
arXiv2011

Why are central radio relics so rare?

In this paper we address the question why cluster radio relics that are connected to shock acceleration, so-called radio gischt, have preferentially been found in the outskirts of galaxy clusters. By identifying merger shock waves in cosmological grid simulations, we explore several prescriptions for relating the energy dissipated in shocks to the energy emitted in the radio band. None of the investigated models produce detectable radio relics within 100-200 kpc from the cluster centre. All models cause > 50 per cent of the detectable relic emission at projected distances > 800 kpc. Central radio relics caused by shocks that propagate along the line-of-sight are rare events for simple geometrical reasons, and they have a low surface brightness making them elusive for current instruments. Our simulations show that the radial distribution of observed relics can be explained by the radial trend of dissipated kinetic energy in shocks, that increases with distance from the cluster centre up until half of the virial radius.

astro-ph.CO↗
arXiv2011

A non-ideal MHD Gadget: Simulating massive galaxy clusters

Magnetic fields in the intra-cluster medium of galaxy clusters have been studied in the past years through different methods. In the next years the up-coming generation of radio telescopes is going to provide new data that have the potential of setting constraints on the properties of magnetic fields in galaxy clusters. Here we present zoomed-in simulations for a set of massive galaxy clusters (M_v > 10^15 M_sun/h). This is an ideal sample to study the evolution of magnetic field during the process of structure formation in detail. Turbulent motions of the gas within the ICM will manifest themselves in a macroscopic magnetic resistivity eta_m, which has to be taken explicitly into account, especially at scales below the resolution limit. We have adapted the MHD GADGET code by Dolag & Stasyszyn (2009) to include the treatment of the magnetic resistivity and for the first time we have included non-ideal MHD equations to better follow the evolution of the magnetic field within galaxy clusters. We investigate which value of the magnetic resistivity eta_m is required to match the magnetic field profile derived from radio observations. We find that a value of eta_m ~ 6 10^27 cm^2/s is necessary to recover the shape of the magnetic field profile inferred from radio observations of the Coma cluster. This value agrees well with the expected level of turbulent motions within the ICM at our resolution limit. The magnetic field profiles can be fitted by a beta-model like profile (Cavaliere & Fusco-Femiano 1976), with small dispersion of the parameters. We find also that that the temperature, density and entropy profiles of the clusters depend on the magnetic resistivity constant, having flatter profiles in the inner regions when the magnetic resistivity increases.

astro-ph.CO↗
arXiv2011

Fractional polarization and magnetic fields in the ICM

The aim of this work is to investigate the average properties of the intra-cluster medium (ICM) magnetic fields, and to search for possible correlations with the ICM thermal properties and cluster radio emission. We have selected a sample of 39 massive galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample, and used Northern VLA Sky Survey data to analyze the fractional polarization of radio sources out to 10 core radii from the cluster centers. Following Murgia et al (2004), we have investigated how different magnetic field strengths affect the observed polarized emission of sources lying at different projected distances from the cluster center. In addition, statistical tests are performed to investigate the fractional polarization trends in clusters with different thermal and non-thermal properties. We find a trend of the fractional polarization with the cluster impact parameter, with fractional polarization increasing at the cluster periphery and decreasing toward the cluster center. Such trend can be reproduced by a magnetic field model with central value of few $μ$G. The logrank statistical test indicates that there are no differences in the depolarization trend observed in cluster with and without radio halo, while the same test indicates significant differences when the depolarization trend of sources in clusters with and without cool core are compared. The comparison between clusters with high and low temperatures does not yields significant differences. Although therole of the gas density should be better accounted for, these results give important indications for models that require a role of the ICM magnetic field to explain the presence of cool core and radio halos in galaxy clusters.

astro-ph.CO↗
arXiv2011

X-ray mass proxies from hydrodynamic simulations of galaxy clusters (paper I)

We present a detailed study of scaling relations between total cluster mass and three mass proxies based on X-ray observables: temperature of the intra-cluster medium, gas mass and the product of the two, Y_X. Our analysis is based on two sets of high-resolution hydrodynamical simulations performed with the TreePM-SPH GADGET code. The first set includes about 140 clusters with masses above 5x10^13 M_sun/h (30 having mass above 10^15 M_sun/h), that have been simulated with (i) non-radiative physics and including (ii) cooling, star formation, chemical enrichment and the effect of supernova feedback triggering galactic ejecta. This large statistics is used to quantify the robustness of the scaling relations, to determine their redshift evolution and to calibrate their intrinsic scatter and its distribution. We use a smaller set of clusters including 18 halos with masses above 5x10^13 M_sun/h to test the robustness of mass proxies against changing the physical processes included in simulations (thermal conduction, artificial viscosity, cooling and star formation, galactic winds and AGN feedback). We find the M-Y_X scaling relation to be the least sensitive one to variations of the ICM physics, with its slope and redshift evolution close to the self-similar model predictions. The distribution of the scatter around the best-fitting relations is close to a log-normal one. M_gas has the smallest scatter in mass, with values of sigma_lnM = 0.04-0.06, depending on the physics included in the simulation, and with a mild dependence on redshift. The M-T relation is the one with the largest scatter, with sigma_lnM > 0.1 at z=0, increasing to > 0.15 at z=1. The intrinsic scatter in the M-Y_X relation is slightly larger than in the M-M_gas relation. These results confirm that both Y_X and M_gas mass proxies are well suited for cosmological applications of future large X-ray surveys. [abridged]

astro-ph.CO↗
arXiv2011

The large-scale diffuse radio emission in A781

A781 belongs to a complex system characterized by extended X-ray emissions that may form part of line of clusters of galaxies along a filament. The aim of this work is to investigate the possible presence of extended, diffuse synchrotron radio emission connected to the intra-cluster medium of A781. We studied the radio continuum emission and the spectral index properties in proximity of the A781 by analyzing archival Very Large Array observations at 1400 and 325 MHz. The main cluster of the system is permeated by diffuse low-surface brightness radio emission which is classified, being located close to the center, as a radio halo. The diffuse emission presents the typical extension and radio power of the other halos known in the literature. Interestingly, the radio halo appears to be linked to a peripheral patch previously found in the literature. The spectrum of this peripheral emission shows a radial steepening which may confirm that this source is indeed a cluster relic.

astro-ph.CO↗
arXiv2010

Galaxy cluster magnetic fields from radio polarized emission

The presence of magnetic fields in the intra-cluster medium of galaxy clusters is now well estabilished. It is directly revealed by the presence of cluster-wide radio sources: radio halos and radio relics. In the last years increasing attention has been devoted to the intra cluster magnetic field through the study of polarized radio emission of radio galaxies, radio halos and radio relics. Recent radio observations have revealed important features of the intra-cluster magnetic field, allowing us to constrain its main properties and to understand the physical processes taking place in the intra-cluster medium. I will review the newest results on galaxy cluster magnetic fields, both focusing on single objects and aimed at describing the magnetic field general properties. The up-coming generation of radio telescopes, EVLA and LOFAR, will shed light on several problematics regarding the cluster magnetic fields and the related non-thermal emission.

astro-ph.CO↗
arXiv2010

Rotation Measures of Radio Sources in Hot Galaxy Clusters

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. 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. 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 (sigmaRM), mostly because of the higher gas density in these clusters. Although the previously known relation between the clusters X-ray surface brightness (Sx) at the radio galaxy location and sigmaRM is confirmed, a possible connection between the sigmaRM-Sx 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.

astro-ph.CO↗
arXiv2010

The intracluster magnetic field power spectrum in Abell 665

The goal of this work is to investigate the power spectrum of the magnetic field associated with the giant radio halo in the galaxy cluster A665. For this, we present new deep Very Large Array total intensity and polarization observations at 1.4 GHz. We simulated Gaussian random three-dimensional turbulent magnetic field models to reproduce the observed radio halo emission. By comparing observed and synthetic radio halo images we constrained the strength and structure of the intracluster magnetic field. We assumed that the magnetic field power spectrum is a power law with a Kolmogorov index and we imposed a local equipartition of energy density between relativistic particles and field. Under these assumptions, we find that the radio halo emission in A665 is consistent with a central magnetic field strength of about 1.3 micro-G. To explain the azimuthally averaged radio brightness profile, the magnetic field energy density should decrease following the thermal gas density, leading to an averaged magnetic field strength over the central 1 Mpc^3 of about 0.75 micro-G. From the observed brightness fluctuations of the radio halo, we infer that the outer scale of the magnetic field power spectrum is ~450 kpc, and the corresponding magnetic field auto-correlation length is ~100 kpc.

astro-ph.CO↗
arXiv2010

The Coma cluster magnetic field from Faraday rotation measures

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. 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 B_0 and radial power-law slope eta, where eta indicates how the field scales with respect to the gas density profile. 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 (B_0=3.9 microG; eta=0.4) and (B_0=5.4 microG; eta=0.7) within 1sigma. The best agreement between observations and simulations is achieved for B_0=4.7 microG; eta=0.5. Values of B_0>7 microG and <3 microG as well as eta < 0.2 and eta > 1.0 are incompatible with RM data at 99 % confidence level.

astro-ph.CO↗
arXiv2010

The diffuse radio filament in the merging system ZwCl 2341.1+0000

In some clusters of galaxies, a diffuse non-thermal emission is present, not obviously associated with any individual galaxy. These sources have been identified as relics, mini-halos, and halos according to their properties and position with respect to the cluster center. Moreover in a few cases have been reported the existence of a diffuse radio emission not identified with a cluster, but with a large scale filamentary region. The aim of this work is to observe and discuss the diffuse radio emission present in the complex merging structure of galaxies ZwCl 2341.1+0000. We have obtained VLA observations at 1.4 GHz to derive a deep radio image of the diffuse emission. Low resolution VLA images show a diffuse radio emission associated to the complex merging region with a largest size = 2.2 Mpc. In addition to the previously reported peripheral radio emission, classified as a double relic, diffuse emission is detected along the optical filament of galaxies. The giant radio source discussed here shows that magnetic fields and relativistic particles are present also in filamentary structures. Possible alternate scenarios are: a giant radio halo in between two symmetric relics, or the merging of two clusters both hosting a central radio halo.

astro-ph.CO↗
arXiv2009

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

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 named radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume. 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. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos. 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 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. The sample of radio halos discussed here represents the population of radio halos observable with present radio telescopes. The 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.

astro-ph.CO↗
arXiv2009

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.

astro-ph.CO↗