arXiv subjects

G. Brunetti

Publications and source records attributed to G. Brunetti.

At least 289 records · Page 16Linked to original sources

arXiv2008

NIR/Optical Counterparts of Hotspots in Radio Galaxies

We present new high spatial resolution VLT and VLA observations of a sample of nine low-power (P_{1.4 GHz} < 10^{25} W/Hz) radio hotspots. Infrared/optical emission is definitely detected in four of the nine observed objects, resulting in a detection rate of at least 45%. This emission is interpreted as synchrotron radiation from the electrons accelerated in the hot spots. The integrated spectra of these hotspots reveal typical break frequencies between 10^5 and 10^6 GHz, two orders of magnitude higher than typically found in high-power hotspots. This supports the idea that in low-power hotspots with their relatively low magnetic field strengths electrons emit most of their energy at higher frequencies. A simple spectral ageing analysis would imply that the emitting electrons have been injected into the hotspot volume less than ~10^3 years ago. We discuss possible scenarios to explain the lack of older electrons in the hotspot region. In particular, the extended morphology of the NIR/optical emission would suggest that efficient re-acceleration mechanisms rejuvenate the electron populations.

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arXiv2008

Non-thermal emission from Galaxy Clusters and future observations with the FERMI gamma-ray telescope and LOFAR

FERMI (formely GLAST) and LOFAR will shortly provide crucial information on the non-thermal components (relativistic particles and magnetic field) in galaxy clusters. After discussing observational facts that already put constraints on the properties and origin of non-thermal components, I will report on the emission spectrum from galaxy clusters as expected in the context of general calculations in which relativistic particles (protons and secondary electrons due to proton-proton collisions) interact with MHD turbulence generated in the cluster volume during cluster-cluster mergers. In this scenario (known as re-acceleration scenario) diffuse cluster-scale radio emission is produced in massive clusters during merging events, while gamma ray emission, at some level, is expected to be common in clusters. Expectations of interest for LOFAR and FERMI are also briefly discussed.

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arXiv2008

A high resolution view of the jet termination shock in a hot spot of the nearby radio galaxy Pictor A: implications for X-ray models of radio galaxy hot spots

Images made with the VLBA have resolved the region in a nearby radio galaxy, Pictor A, where the relativistic jet that originates at the nucleus terminates in an interaction with the intergalactic medium, a so-called radio galaxy hot spot. This image provides the highest spatial resolution view of such an object to date (16 pc), more than three times better than previous VLBI observations of similar objects. The north-west Pictor A hot spot is resolved into a complex set of compact components, seen to coincide with the bright part of the hot spot imaged at arcsecond-scale resolution with the VLA. In addition to a comparison with VLA data, we compare our VLBA results with data from the HST and Chandra telescopes, as well as new Spitzer data. The presence of pc-scale components in the hot spot, identifying regions containing strong shocks in the fluid flow, leads us to explore the suggestion that they represent sites of synchrotron X-ray production, contributing to the integrated X-ray flux of the hot spot, along with X-rays from synchrotron self-Compton scattering. This scenario provides a natural explanation for the radio morphology of the hot spot and its integrated X-ray emission, leading to very different predictions for the higher energy X-ray spectrum compared to previous studies. From the sizes of the individual pc-scale components and their angular spread, we estimate that the jet width at the hot spot is in the range 70 - 700 pc, which is comparable to similar estimates in PKS 2153-69, 3C 205, and 4C 41.17. The lower limit in this range arises from the suggestion that the jet may dither in its direction as it passes through hot spot backflow material close to the jet termination point, creating a "dentist drill" effect on the inside of a cavity 700 pc in diameter.

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arXiv2008

A morphological comparison between giant radio halos and radio mini--halos in galaxy clusters

In this letter we present a morphological comparison between giant radio halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity, P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for mini--halos: mini--halo clusters share the same region of giant halo clusters in the (P_{1.4},L_X) plane, whereas they are clearly separated in the (P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be more than 50 times larger than that of giant halos, implying a very efficient process for their origins. By assuming a scenario of sporadical turbulent particle re-acceleration for both giant and mini halos, we discuss basic physical differences between these sources. Regardless of the origin of the turbulence, a more efficient source of injection of particles, which eventually takes part in the re-acceleration process, is required in mini-halos, and this may result from the central radio galaxy or from proton-proton collisions in the dense cool core regions.

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arXiv2008

Cosmological Shocks in Eulerian Simulations: Main Properties and Cosmic Rays Acceleration

Aims: morpholgies, number and energy distributions of Cosmological Shock Waves from a set of ENZO cosmological simulations are produced, along with a study of the connection with Cosmic Rays processes in different environments. Method: we perform cosmological simulations with the public release of the PPM code ENZO, adopt a simple and physically motivated numerical setup to follow the evolution of cosmic structures at the resolution of 125kpc per cell, and characterise shocks with a new post processing scheme. Results: we estimate the efficency of the acceleration of Cosmic Ray particles and present the first comparison of our results with existing limits from observations of galaxy clusters.

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arXiv2008

Shock acceleration as origin of the radio relic in A521?

We present new high sensitivity observations of the radio relic in A521 carried out with the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327-610 MHz. To this aim we used the new GMRT observations and other proprietary as well as archival data. We also searched for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of the theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is further supported by our finding of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is likely a shock front.

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arXiv2008

GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4. II.The eBCS clusters and analysis of the complete sample

We present the results of the GMRT cluster radio halo survey. The main purposes of our observational project are to measure which fraction of massive galaxy clusters in the redshift range z=0.2--0.4 hosts a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of the non--thermal radio emission. We selected a complete sample of 50 clusters in the X-ray band from the REFLEX (27) and the eBCS (23) catalogues. In this paper we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for all clusters still lacking high sensitivity radio information, i.e. 16 eBCS and 7 REFLEX clusters, thus completing the radio information for the whole sample. The typical sensitivity in our images is in the range 1$σ\sim 35-100 μ$Jy b$^{-1}$. We found a radio halo in A697, a diffuse peripheral source of unclear nature in A781, a core--halo source in Z7160, a candidate radio halo in A1682 and ``suspect'' central emission in Z2661. Including the literature information, a total of 10 clusters in the sample host a radio halo. A very important result of our work is that 25 out of the 34 clusters observed with the GMRT do not host extended central emission at the sensitivity level of our observations, and for 20 of them firm upper limits to the radio power of a giant radio halo were derived. The GMRT Radio Halo Survey shows that radio halos are not common, and our findings on the fraction of giant radio halos in massive clusters are consistent with the statistical expectations based on the re--acceleration model. Our results favour primary to secondary electron models.

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arXiv2007

Revised statistics of radio halos and the re-acceleration model

Aims. The statistical properties of radio halos can be used to discriminate among the possible models for their origin. Therefore an unbiased and exhaustive investigation in this direction is crucial. Methods. With this goal in mind in this paper we revise the occurrence of radio halos in the redshift range 0-0.4, combining the low redshift (z<0.2) statistical study of XBACs clusters with the NVSS (by Giovannini et al. 1999) with our recent results from the radio follow up of REFLEX and eBCS clusters, the GMRT radio halo survey, at higher redshift (0.2<z<0.4). Results. We find a significant statistical evidence (at 3.7 sigma) of an increase of the fraction of clusters with Radio Halos with the X-ray luminosity (mass) of the parent clusters and show that this increase is in line with statistical calculations based on the re-acceleration scenario. We argue that a fundamental expectation of this scenario is that the probability to have radio halos emitting at hundred MHz is larger than that at GHz frequencies and thus that future radio interferometers operating at low frequencies, such as LOFAR and LWA, should detect a larger number of radio halos with respect to that caught by present GHz observations. We also show that the expected increase of the fraction of clusters with radio halos with the cluster mass as measured with future LOFAR and LWA surveys should be less strong than that in present surveys.

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arXiv2007

Hard X-rays from Galaxy Clusters and SIMBOL-X

Non thermal emission from galaxy clusters demonstrates the existence of relativistic particles and magnetic fields in the Intra Cluster Medium (ICM). Present instruments do not allow to firmly establish the energy associated to these components. In a few years gamma ray observations will put important constraints on the energy content of non thermal hadrons in clusters, while the combination of radio and hard X-ray data will be crucial to measure the energy content in the form of relativistic electrons and magnetic field. SIMBOL-X is expected to drive an important breakthrough in the field also because it is expected to operate in combination with the forthcoming low frequency radio telescopes (LOFAR, LWA). In this contribution we report first estimates of statistical properties of the hard X-ray emission in the framework of the re-acceleration model. This model allows to reproduce present radio data for Radio Halos and to derive expectations for future low frequency radio observations, and thus our calculations provide hints for observational strategies for future radio and hard--X-ray combined observations.

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arXiv2007

Cosmic rays and Radio Halos in galaxy clusters : new constraints from radio observations

Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray luminous, galaxy clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy clusters does not show any hint of Mpc scale synchrotron radio emission at the cluster center (Radio Halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of Radio Halos. Our measurements allow us to derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.

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arXiv2007

An X-ray view of Pictor A radio lobes: a spatially resolved study

A spatially resolved analysis of the lobes of the radio galaxy Pictor A has been performed for the first time starting from a 50 ksec XMM-Newton observation. Magnetic field, B_{IC}, particle density, particle to magnetic field energy density ratios have been measured. Our study shows that B_{IC} varies through the lobes. On the contrary, a rather uniform distribution of the particles is observed. In both the lobes, the equipartition magnetic field, B_{eq}, is bigger than the Inverse Compton value, B_{IC}, calculated from the radio to X-ray flux ratio.

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arXiv2007

Radio Lobes of Pictor A: an X-ray spatially resolved Study

A new XMM observation has made possible a detailed study of both lobes of the radio galaxy Pictor A. Their X-ray emission is of non thermal origin and due to Inverse Compton scattering of the microwave background photons by relativistic electrons in the lobes, as previously found. In both lobes, the equipartition magnetic field (Beq) is bigger than the Inverse Compton value (Bic), calculated from the radio and X-ray flux ratio. The Beq/Bic ratio never gets below 2, in spite of the large number of reasonable assumptions tested to calculate Beq, suggesting a lobe energetic dominated by particles. The X-ray data quality is good enough to allow a spatially resolved analysis. Our study shows that Bic varies through the lobes. It appears to increase behind the hot spots. On the contrary, a rather uniform distribution of the particles is observed. As a consequence, the radio flux density variation along the lobes appears to be mainly driven by magnetic field changes.

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arXiv2007

New scaling relations in cluster radio halos and the re-acceleration model

In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P_R) vs size of the radio emitting region (R_H), and P_R vs total cluster mass (M_H) contained in the emitting region and cluster velocity dispersion (sigma_H) in this region. We search for these correlations by analyzing the most recent radio and X-ray data available in the literature for a well known sample of clusters with giant radio halos. In particular we find a good correlation between P_R and R_H and a very tight ``geometrical'' scaling between M_H and R_H. From these correlations P_R is also expected to scale with M_H and sigma_H and this is confirmed by our analysis. We show that all the observed trends can be well reconciled with expectations in the case of a slight variation of the mean magnetic field strength in the radio halo volume with M_H. A byproduct correlation between R_H and sigma_H is also found, and can be further tested by optical studies. In addition, we find that observationally R_H scales non-linearly with the virial radius of the host cluster, and this immediately means that the fraction of the cluster volume which is radio emitting increases with cluster mass and thus that the non-thermal component in clusters is not self-similar.

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arXiv2007

Compressible Turbulence in Galaxy Clusters: Physics and Stochastic Particle Re-acceleration

We attempt to explain the non-thermal emission arising from galaxy clusters as a result of the re-acceleration of electrons by compressible turbulence induced by cluster mergers. In our model intracluster medium (ICM) is represented by a high beta plasma in which turbulent motions are driven at large scales. The corresponding injection velocities are higher than the Alfven velocity. As a result, the turbulence is approximately isotropic up to the scale at which the turbulent velocity gets comparable with the Alfven velocity. Under the hypothesis that turbulence in the ICM is highly super- Alfvenic the magnetic field is passively advected and the field lines are bended on scales smaller than that of the classical, unmagnetized, ion-ion mean free path. This affects ion diffusion and the strength of the effective viscosity. Under these conditions the bulk of turbulence in hot (5-10 keV temperature) galaxy clusters is likely to be dissipated at collisionless scales via resonant coupling with thermal and fast particles. We use collisionless physics to derive the amplitude of the different components of the energy of the compressible modes, and review and extend the treatment of plasma damping in the ICM. We calculate the acceleration of both protons and electrons taking into account both TTD acceleration and non-resonant acceleration by large scale compressions. We find that relativistic electrons can be re-accelerated in the ICM up to energies of several GeV provided that the rms velocity of the compressible turbulent-eddies is (V_L/c_s)^2~0.15-0.3. We find that under typical conditions ~ 2-5 % of the energy flux of the cascading of compressible motions injected at large scales goes into the acceleration of fast particles and that this may explain the observed non-thermal emission from merging galaxy clusters.

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arXiv2007

Low-Frequency study of two clusters of galaxies: A2744 and A2219

Spectral index images can be used to constraint the energy spectrum of relativistic electrons and magnetic field distribution in radio halos and relics, providing useful information to understand their formation, evolution and connection to cluster merger processes. We present low-frequency images of the two clusters of galaxies: A2744 and A2219, in which a wide diffuse emission is detected. Observations were made with the Very Large Array at the frequency of 325 MHz. For both clusters deep Very Large Array 1.4 GHz observations are available. Combining the 325 MHz and 1.4 GHz data, we obtained the spectral index images and the brightness radial profiles of the diffuse radio emission with a resolution of ~ 1'. The azimuthally averaged spectral index in A2744 is constant to a value close to alpha ~ 1 up to a distance of 1 Mpc from the cluster center. However, the spectral index image shows the presence of localized regions in which the radio spectrum is significantly different from the average. The observed spectral index variations range from a minimum of alpha ~ 0.7 +/- 0.1 to a maximum alpha ~ 1.5 +/- 0.2. From the comparison of the spectral index with the X-rays data it is found for the first time that the flat spectrum regions of the radio halo tend to have higher temperature. In the case of A2219, the radio emission in the central regions of the cluster is dominated by the blend of discrete sources. The azimuthally averaged radio spectrum is alpha ~ 0.8 in the central region of the cluster and is close to a value of alpha ~ 1 in the radio halo. The limited sensitivity of the 325 MHz image does not allowed us to detect all the radio halo structure seen at 1.4 GHz and therefore no constrains on the point-to-point variations of the spectral index have been obtained for this cluster.

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arXiv2006

GMRT Radio Halo Survey in galaxy clusters at z = 0.2 -- 0.4 I.The REFLEX sub--sample

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

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arXiv2006

Statistics of Giant Radio Halos from Electron Reacceleration Models

The most important evidence of non-thermal phenomena in galaxy clusters comes from Giant Radio Halos (GRHs), synchrotron radio sources extended over Mpc scales, detected in a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by "in situ" stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano & Brunetti (2005) have recently shown that the expected fraction of clusters with GRHs and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 % of the turbulence in clusters is in the form of compressible modes. In this work we extend these calculations by including a scaling of the magnetic field strength with cluster mass. We show that the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B \propto M^b, with b>0.5 and typical micro Gauss strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained. The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P_{1.4 GHz} 10^{24} W/Hz, and severely cut-off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters. We expect a total number of GRHs to be discovered at ~mJy radio fluxes of ~100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the fortcoming operation of low frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

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arXiv2006

Turbulent Velocity Fields in SPH--simulated Galaxy Clusters

We present a study of the turbulent velocity fields in the Intra Cluster Medium of a sample of 21 galaxy clusters simulated by the SPH--code Gadget2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximatively with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi--analytical calculations, and the combination between the two methods allows to constrain the scaling law over more than two decades in cluster mass.

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