arXiv subjects

G. Brunetti

Publications and source records attributed to G. Brunetti.

At least 307 records · Page 17Linked to original sources

arXiv2006

Constraining B in galaxy clusters from statistics of giant radio halos

There are several possibilities to constrain the value of the magnetic field in the ICM, the most direct ones being the combination of inverse Compton and synchrotron observations, and the Faraday rotation measures. Here we discuss on the possibility to provide constraints on the magnetic field in the ICM from the analysis of the statistical properties of the giant radio halos, Mpc--scale diffuse radio emission in galaxy clusters. Present observations of a few well studied radio halos can be interpreted under the hypothesis that the emitting relativistic electrons are re-accelerated on their way out. By using statistical calculations carried out in the framework of the re-acceleration model we show that the observed radio--power vs cluster mass correlation in radio halos can be reproduced only by assuming $μ$G fields in the ICM and a scaling of the magnetic field with cluster mass $B \propto M_v^b$, with $b \geq 0.6$. We also show that the expected occurrence of radio halos with mass and redshift, and their number counts are sensitive to the magnetic field intensity in massive galaxy clusters and to the scaling of B with cluster mass. Thus future deep surveys of radio halos would provide constraints on B in galaxy clusters.

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arXiv2005

The cluster relic source in A521

We present high sensitivity radio observations of the merging cluster A521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of $\sim$1 square degree, with a sensitivity limit of $1σ$ = 35 $μ$Jy b$^{-1}$. The most relevant result of these observations is the presence of a radio relic at the cluster periphery, at the edge of a region where group infalling into the main cluster is taking place. Thanks to the wealth of information available in the literature in the optical and X-ray bands, a multi--band study of the relic and its surroundings was performed. Our analysis is suggestive of a connection between this source and the complex ongoing merger in the A521 region. The relic might be ``revived' fossil radio plasma through adiabatic compression of the magnetic field or shock re--acceleration due to the merger events. We also briefly discussed the possibility that this source is the result of induced ram pressure stripping of radio lobes associated with the nearby cluster radio galaxy J0454--1016a. Allowing for the large uncertainties due to the small statistics, the number of radio emitting early--type galaxies found in A521 is consistent with the expectations from the standard radio luminosity function for local (z$\le$0.09) cluster ellipticals.

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arXiv2005

A new XMM-Newton observation of Pictor A radio lobes confirms the non-thermal origin of the X-ray emission

X-ray emission from the eastern radio lobe of the FRII Radio Galaxy Pictor A was serendipitously discovered by a short observation of XMM-Newton in 2001. The X-ray spectrum, accumulated on a region covering about half of the entire radio lobe, was well described by both a thermal model and a power law, making non-univocal the physical interpretation. A new XMM-Newton observation performed in 2005 has allowed the detection of the X-ray emission from both radio lobes and unambiguously revealed its non-thermal origin. The X-ray emission is due to Inverse Compton (IC) of the cosmic microwave background photons by relativistic electrons in the lobe. We confirm the discrepancy between the magnetic field, as deduced from the comparison of the IC X-ray and radio fluxes, and the equipartition value.

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arXiv2005

Alfvenic reacceleration of relativistic particles in galaxy clusters in the presence of secondary electrons and positrons

In a previous paper (Brunetti et al. 2004) we presented the first self-consistent calculations of the time-dependent coupled equations for the electrons, hadrons and Alfvén waves in the intracluster medium, which describe the stochastic acceleration of the charged particles and the corresponding spectral modification of the waves. Under viable assumptions, this system of mutually interacting components was shown to accurately describe several observational findings related to the radio halos in clusters of galaxies. In this paper, we add to the self-consistency of the calculations by including the generation and re-energization of secondary electrons and positrons, produced by the inelastic interactions of cosmic rays with the thermal gas in the intracluster medium. The bulk of Cosmic rays is expected to be confined within the cluster volume for cosmological times, so that the rate of production of secondary electrons, as well as gamma rays, may become correspondingly enhanced. If MHD waves are present, as it may be expected in the case of a recent merger event, then the reacceleration of secondary electrons and positrons can significantly affect the phenomenology of the non thermal processes in clusters. We investigate here these effects for the first time.

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arXiv2005

Turbulent gas motions in galaxy cluster simulations: The role of SPH viscosity

Smoothed particle hydrodynamics (SPH) employs an artificial viscosity to properly capture hydrodynamical shock waves. In its original formulation, the resulting numerical viscosity is large enough to suppress structure in the velocity field on scales well above the nominal resolution limit, and to damp the generation of turbulence by fluid instabilities. This could artificially suppress random gas motions in the intracluster medium (ICM), which are driven by infalling structures during the hierarchical structure formation process. We show that this is indeed the case by analysing results obtained with an SPH formulation where an individual, time-variable viscosity is used for each particle (Monaghan 1997). Using test calculations involving strong shocks, we demonstrate that this scheme captures shocks as well as the original formulation of SPH, but, in regions away from shocks, the numerical viscosity is much smaller. In a set of nine high-resolution simulations of cosmological galaxy cluster formation, we find that this low--viscosity formulation of SPH produces substantially higher levels of turbulent gas motions in the ICM, reaching a kinetic energy content in random gas motions (measured within a 1Mpc cube) of up to 5%-30% of the thermal energy content, depending on cluster mass. This has also significant effects on radial gas profile. We find a central flattening of the entropy profile and a reduction of the central gas density in the low--viscosity scheme. Interestingly, this tends to reduce the differences seen in SPH and adaptive mesh refinement simulations of cluster formation. Finally, invoking a model for particle acceleration by MHD waves driven by turbulence, we find efficient electron acceleration to power diffuse radio emission.

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arXiv2005

Spectral properties and origin of the radio halo in A3562

We present a new detailed multiband study of the merging cluster A3562, in the core of the Shapley Concentration Supercluster. We analyzed new, low frequency radio data performed at 240 MHz, 332 MHz and 610 MHz with the Giant Metrewave Radio Telescope (GMRT). The new GMRT data allowed us to carry out a detailed study of the radio halo at the centre of A3562, as well as of the head--tail radio galaxy J1333--3141 embedded in it, and of the extended emission around the peripheral cluster galaxy J1332--3146a. Thanks to the present observations we could derive the integrated spectrum of the radio halo with five data points in the frequency range 240 MHz -- 1.4 GHz. Our data show a clear steepening of the total spectrum in this frequency range. Furthermore, by comparing the GMRT 332 MHz image with a previously published VLA 1.4 GHz image, we produced an image of the halo spectral index distribution. The image shows a very complex structure, with an average value of $α^{1.4GHz}_{332MHz} \sim 1.5$ and a number of knots steepening up to $\sim 2$. We performed a combined morphological and statistical analysis using the radio images and the quantities derived from XMM--Newton and {\it Chandra} observations. We discuss our results in the light of particle re--acceleration processes in galaxy clusters. In particular, we outline an overall picture, consistent with the available radio and X-ray data, in which the cluster merger kinematics, the injection of turbulence and B--amplification induced by the merger between A 3562 and SC 1329--313 are jointly taken into account.

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arXiv2005

Bow shock and radio halo in the merging cluster A520

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M=2.1+0.4-0.3. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide pre-energized electrons for subsequent turbulent re-acceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of 1.2 right behind the edge with quick steepening further away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of pre-existing relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such pre-shock emission, then the electrons are indeed accelerated (or re-accelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.

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arXiv2004

Cluster mergers and non-thermal phenomena: expectations from a statistical magneto-turbulent model

The most important evidences for non-thermal phenomena in galaxy clusters comes from the spectacular synchrotron radio emission diffused over Mpc scale observed in a growing number of massive clusters. A promising possibility to explain giant radio halos 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 to investigate the connection between thermal and non-thermal properties of the ICM, in this paper we develope 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 halos 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.

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arXiv2004

Properties and Spectral Behaviour of Cluster Radio Halos

Several arguments have been presented in the literature to support the connection between radio halos and cluster mergers. The spectral index distributions of the halos in A665 and A2163 provide a new strong confirmation of this connection, i.e. of the fact that the cluster merger plays an important role in the energy supply to the radio halos. Features of the spectral index (flattening and patches) are indication of a complex shape of the radiating electron spectrum, and are therefore in support of electron reacceleration models. Regions of flatter spectrum are found to be related to the recent merger. In the undisturbed cluster regions, instead, the spectrum steepens with the distance from the cluster center.The plot of the integrated spectral index of a sample of halos versus the cluster temperature indicates that clusters at higher temperature tend to host halos with flatter spectra. This correlation provides further evidence of the connection between radio emission and cluster mergers.

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arXiv2004

Anisotropic inverse Compton emission in the radio galaxy 3C 265

We present the results from a Chandra observation of the powerful radio galaxy 3C 265. We detect X-ray emission from the nucleus, the radio hotspots and lobes. In particular, the lobe X-ray emission is well explained as anisotropic inverse Compton scattering of the nuclear photons by the relativistic electrons in the radio lobes; the comparison between radio synchrotron and IC emission yields a magnetic field strength a factor about 2 lower than that calculated under minimum energy conditions. The X-ray spectrum of the nucleus is consistent with that of a powerful, strongly absorbed quasar and the X-ray emission of the south-eastern hotspot can be successfully reproduced by a combination of synchro-self Compton and inverse Compton emission assuming a magnetic field slightly lower than equipartition.

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arXiv2004

Radio halos in merging clusters of galaxies

We present the preliminary results of 235 MHz, 327 MHz and 610 MHz observations of the galaxy cluster A3562 in the core of the Shapley Concentration. The purpose of these observations, carried out with the Giant Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo located at the centre of A3562 and determine the shape of its radio spectrum at low frequencies, in order to understand the origin of this source. In the framework of the re--acceleration model, the preliminary analysis of the halo spectrum suggests that we are observing a young source (few $10^8$ yrs) at the beginning of the re--acceleration phase.

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arXiv2004

Cluster Mergers, Radio Halos and Hard X-ray Tails: A Statistical Magneto-Turbulent Model

There is now firm evidence that the ICM consists of a mixture of hot plasma, magnetic fields and relativistic particles. The most important evidences for non-thermal phenomena in galaxy clusters comes from the diffuse Mpc-scale synchrotron radio emission (radio halos) observed in a growing number of massive clusters (Feretti 2003) and from hard X-ray (HXR) excess emission (detected in a few cases) which can be explained in terms of IC scattering of relativistic electrons off the cosmic microwave background photons (Fusco-Femiano et al. 2003). There are now growing evidences that giant radio halos may be naturally accounted for by synchrotron emission from relativistic electrons reaccelerated by some kind of turbulence generated in the cluster volume during merger events (Brunetti 2003). With the aim to investigate the connection between thermal and non-thermal properties of the ICM, we have developed a statistical magneto-turbulent model which describes the evolution of the thermal and non-thermal emission from clusters. We calculate the energy and spectrum of the magnetosonic waves generated during cluster mergers, the acceleration and evolution of relativistic electrons and thus the resulting synchrotron and inverse Compton spectra. Here we give a brief description of the main results, while a more detailed discussion will be presented in a forthcoming paper. Einstein-De Sitter cosmology, $H_o=50$ km $s^{-1}$$Mpc^{-1}$, $q_o=0.5$, is assumed.

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arXiv2004

Spectral index maps of the radio halos in Abell 665 and Abell 2163

New radio data at 330 MHz are presented for the rich clusters Abell 665 and Abell 2163, whose radio emission is characterized by the presence of a radio halo. These images allowed us to derive the spectral properties of the two clusters under study. The integrated spectra of these halos between 0.3 GHz and 1.4 GHz are moderately steep: alfa(0.3-1.4) = 1.04 and = 1.18, for A665 and A2163, respectively. The spectral index maps, produced with an angular resolution of the order of 1 arcmin, show features of the spectral index (flattening and patches), which are indication of a complex shape of the radiating electron spectrum, and are therefore in support of electron reacceleration models. Regions of flatter spectrum are found to be related to the recent merger activity in these clusters. This is the first strong confirmation that the cluster merger supplies energy to the radio halo. In the undisturbed cluster regions, the spectrum steepens with the distance from the cluster center. This is interpreted as the result of the combination of the magnetic field profile with the spatial distribution of the reacceleration efficiency, thus allowing us to set constraints on the radial profile of the cluster magnetic field.

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arXiv2003

In-Situ Particle Acceleration in Extragalactic Radio Hot Spots: Observations Meet Expectations

We discuss, in terms of particle acceleration, the results from optical VLT observations of hot spots associated with radio galaxies. On the basis of observational and theoretical grounds, it is shown that: 1. relatively low radio-radio power hot spots are the optimum candidates for being detected at optical waves. This is supported by an unprecedented optical detection rate of 70% out of a sample of low radio power hot spots. 2. the shape of the synchrotron spectrum of hot spots is mainly determined by the strength of the magnetic field in the region. In particular, the break frequency, related to the age of the oldest electrons in the hot spots, is found to increase with decreasing synchrotron power and magnetic field strength. Both observational results are in agreement with an in-situ particle acceleration scenario.

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arXiv2003

Inverse Compton X-rays from the radio galaxy 3C 219

We report the results from a Chandra observation of the powerful nearby (z=0.1744) radio galaxy 3C 219. We find evidence for non-thermal X-ray emission from the radio lobes which fits fairly well with a combination of inverse Compton scattering of Cosmic Microwave Background radiation and of nuclear photons with the relativistic electrons in the lobes. The comparison between radio synchrotron and IC emission yields a magnetic field strength significantly lower (about a factor 3) than that calculated under minimum energy conditions; the source energetics is then dominated by the relativistic particles.

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arXiv2003

The radio halo in the merging cluster A3562

we present new VLA observations at 1.4 GHz confirming the presence of a radio halo at the centre of the cluster A3562, in the core of the Shapley Concentration. We also report a detailed multifrequency radio study of the head tail galaxy J1333--3141, which is completely embedded in the halo emission. The radio halo has an irregular shape, and a largest linear size of $\sim$ 620 kpc, which is among the smallest found in the literature. The source has a steep spectrum, i.e. $α_{843 MHz}^{1.4 GHz} \sim 2$, and its total radio power, P$_{1.4 GHz} \sim 2 \times10^{23}$ W Hz$^{-1}$, is the lowest known to date.The radio power of the halo and the X-ray parameters of the cluster, such as L$_X$ and kT, nicely fit the correlations found in the literature for the other halo clusters, extending them to low radio powers. We found that the total number of electrons injected in the cluster environment by the head--tail source is enough to feed the halo, if we assume that the galaxy has been radio active over a large fraction of its crossing time. We discuss possible origins of the radio halo in the light of the two--phase model (Brunetti et al. 2001) and propose that the observed scenario is the result of a young source at the beginning of the reacceleration phase.

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arXiv2002

Modelling the non-thermal emission from galaxy clusters

We discuss the relevant processes for the relativistic electrons in the ICM and the possible mechanisms responsible for the production of these electrons. We focus on the origin of the radio halos giving some of the observational diagnostics which may help in discriminating among the different models proposed so far. Finally, we briefly discuss the discrepancy between the value of the magnetic field assuming an inverse Compton (IC) origin of the hard X-ray emission (HXR) and that obtained from Faraday Rotation Measurements (RM).

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arXiv2002

Chandra detects inverse Compton emission from the radio galaxy 3C 219

We report the results from a {\it Chandra} observation of the powerful nearby (z=0.1744) radio galaxy 3C 219. We find evidence for non--thermal X-ray emission from the radio lobes which fits fairly well with a combination of IC scattering of CMB and of nuclear photons. The comparison between radio synchrotron and IC fluxes yields a magnetic field strength significantly lower ($\sim 3$) than that calculated under minimum energy conditions; the majority of the energetics is then associated to the relativistic particles.

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