arXiv subjects

R. Cassano

Publications and source records attributed to R. Cassano.

At least 127 records · Page 7Linked to original sources

arXiv2010

Radio Halos and the importance of future observations at low frequency

Radio observations discovered large scale non thermal sources in the central Mpc regions of dynamically disturbed galaxy clusters (radio halos). The morphological and spectral properties of these sources suggest that the emitting electrons are accelerated by spatially distributed and gentle mechanisms, providing some indirect evidence for turbulent acceleration in the inter-galactic-medium (IGM). Radio and X-ray surveys allow to investigate the statistics of radio halos and unveil a bimodal behaviour of the radio properties of galaxy clusters: merging clusters host radio halos and trace the well known radio--X correlation, while more relaxed clusters do not host radio halos and populate a region well separated from that spanned by the above correlation. This appears consistent with the hypothesis that relativistic electrons can be reaccelerated by MHD turbulence generated during cluster mergers. In the context of this model the population of radio halos consists of a mixture of halos with different spectral properties, most of them with very steep spectrum and visible only at low radio frequencies. For this reason the future LOFAR surveys may provide a robust test to this theoretical hypothesis.

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arXiv2010

On the connection between giant radio halos and cluster mergers

The frequently observed association between giant radio halos and merging galaxy clusters has driven present theoretical models of non-thermal emission from galaxy clusters, which are based on the idea that the energy dissipated during cluster-cluster mergers could power the formation of radio halos. To quantitatively test the merger-halo connection we present the first statistical study based on deep radio data and X-ray observations of a complete X-ray selected sample of galaxy clusters with X-ray luminosity > 5x 10^44 erg/s and redshift 0.2<z< 0.32. Using several methods to characterize cluster substructures, namely the power ratios, centroid shift and X-ray brightness concentration parameter, we show that clusters with and without radio halo can be quantitatively differentiated in terms of their dynamical properties. In particular, we confirm that radio halos are associated to dynamically disturbed clusters and cluster without radio halo are more "relaxed", with only a couple of exceptions where a disturbed cluster does not exhibit a halo.

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arXiv2010

Radio Halos From Simulations And Hadronic Models II: The Scaling Relations of Radio Halos

We use results from a constrained, cosmological MHD simulation of the Local Universe to predict radio halos and their evolution for a volume limited set of galaxy clusters and compare to current observations. The simulated magnetic field inside the clusters is a result of turbulent amplification within them, with the magnetic seed originating from star-burst driven, galactic outflows. We evaluate three models, where we choose different normalizations for the Cosmic Ray proton population within clusters. Similar to our previous analysis of the Coma cluster (Donnert et al. 2010), the radial profile and the morphological properties of observed radio halos can not be reproduced, even with a radially increasing energy fraction within the cosmic ray proton population. Scaling relations between X-ray luminosity and radio power can be reproduced by all models, however all models fail in the prediction of clusters with no radio emission. Also the evolutionary tracks of our largest clusters in all models fail to reproduce the observed bi-modality in radio luminosity. This provides additional evidence that the framework of hadronic, secondary models is disfavored to reproduce the large scale diffuse radio emission of galaxy clusters. We also provide predictions for the unavoidable emission of $γ$-rays from the hadronic models for the full cluster set. None of such secondary models is yet excluded by the observed limits in $γ$-ray emission, emphasizing that large scale diffuse radio emission is a powerful tool to constrain the amount of cosmic ray protons in galaxy clusters.

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arXiv2009

Unveiling radio halos in galaxy clusters in the LOFAR era

Giant radio halos are mega-parsec scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. Radio halos provide one of the most important pieces of evidence for non-thermal components in large scale structure. Statistics of their properties can be used to discriminate among various models for their origin. Therefore, theoretical predictions of the occurrence of radio halos are important as several new radio telescopes are about to begin to survey the sky at low frequencies with unprecedented sensitivity. In this paper we carry out Monte Carlo simulations to model the formation and evolution of radio halos in a cosmological framework. We extend previous works on the statistical properties of radio halos in the context of the turbulent re-acceleration model. First we compute the fraction of galaxy clusters that show radio halos and derive the luminosity function of radio halos. Then, we derive differential and integrated number count distributions of radio halos at low radio frequencies with the main goal to explore the potential of the upcoming LOFAR surveys. By restricting to the case of clusters at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is expected to detect about 350 giant radio halos. About half of these halos have spectral indices larger than 1.9 and substantially brighten at lower frequencies. If detected they will allow for a confirmation that turbulence accelerates the emitting particles. We expect that also commissioning surveys, such as MSSS, have the potential to detect about 60 radio halos in clusters of the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will allow us to constrain how the rate of formation of radio halos in these clusters depends on cluster mass.

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arXiv2009

A giant radio halo in the massive and merging cluster Abell 1351

We report on the detection of diffuse radio emission in the X-ray luminous and massive galaxy cluster A1351 (z=0.322) using archival Very Large Array data at 1.4 GHz. Given its central location, morphology, and Mpc-scale extent, we classify the diffuse source as a giant radio halo. X-ray and weak lensing studies show A1351 to be a system undergoing a major merger. The halo is associated with the most massive substructure. The presence of this source is explained assuming that merger-driven turbulence may re-accelerate high-energy particles in the intracluster medium and generate diffuse radio emission on the cluster scale. The position of A1351 in the logP$_{1.4 GHz}$ - logL$_{X}$ plane is consistent with that of all other radio-halo clusters known to date, supporting a causal connection between the unrelaxed dynamical state of massive ($>10^{15} M_{\odot}$) clusters and the presence of giant radio halos.

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arXiv2009

On the evolution of giant radio halos and their connection with cluster mergers

Giant radio halos are diffuse, Mpc-scale, synchrotron sources located in the central regions of galaxy clusters and provide the most relevant example of cluster non-thermal activity. Radio and X-ray surveys allow to investigate the statistics of halos and may contribute to constrain their origin and evolution. We investigate the distribution of clusters in the plane X-ray (thermal, L_X) vs synchrotron (P_{1.4})luminosity, where clusters hosting giant radio halos trace the P_{1.4}--L_X correlation and clusters without radio halos populate a region that is well separated from that spanned by the above correlation. The connection between radio halos and cluster mergers suggests that the cluster Mpc-scale synchrotron emission is amplified during these mergers and then suppressed when clusters become more dynamically relaxed. In this context, by analysing the distribution in the P_{1.4}--L_X plane of clusters from X-ray selected samples with adequate radio follow up, we constrain the typical time-scale of evolution of diffuse radio emission in clusters and discuss the implications for the origin of radio halos. We conclude that cluster synchrotron emission is suppressed (and amplified) in a time-scale significantly smaller than 1 Gyr. We show that this constraint appears difficult to reconcile with the hypothesis that the halo's radio power is suppressed due to dissipation of magnetic field in galaxy clusters. On the other hand, in agreement with models where turbulent acceleration plays a role, present constraints suggest that relativistic electrons are accelerated in Mpc-scale regions, in connection with cluster mergers and for a time-interval of about 1 Gyr, and then they cool in a relatively small time-scale, when the hosting cluster becomes more dynamically relaxed.

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arXiv2009

Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323

We present a combined radio, X-ray and optical study of the galaxy cluster RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and distant giant radio halos known to date, suggesting that it may be undergoing a strong merger process. The aim of our multiwavelength study is to investigate the radio-halo cluster merger scenario. We studied the radio properties of the giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4 GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio Telescope, in combination with previously published GMRT data at 610 MHz. The dynamical state of the cluster was investigated by means of X-ray Chandra observations and optical ESO--NTT observations. Our study confirms that RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy morphology of the radio halo could be due to a combination of the filamentary structure of the magnetic field and turbulence in the inital stage of a cluster merger.

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arXiv2009

Radio Halos From Simulations And Hadronic Models I: The Coma cluster

We use the results from a constrained, cosmological MHD simulation of the Local Universe to predict the radio halo and the gamma-ray flux from the Coma cluster and compare it to current observations. The simulated magnetic field within the Coma cluster is the result of turbulent amplification of the magnetic field during build-up of the cluster. The magnetic seed field originates from star-burst driven, galactic outflows. The synchrotron emission is calculated assuming a hadronic model. We follow four approaches with different distributions for the cosmic-ray proton (CRp) population within galaxy clusters. The radial profile the radio halo can only be reproduced with a radially increasing energy fraction within the cosmic ray proton population, reaching $>$100% of the thermal energy content at $\approx$ 1Mpc, e.g. the edge of the radio emitting region. Additionally the spectral steepening of the observed radio halo in Coma cannot be reproduced, even when accounting for the negative flux from the thermal SZ effect at high frequencies. Therefore the hadronic models are disfavored from present analysis. The emission of $γ$-rays expected from our simulated coma is still below the current observational limits (by a factor of $\sim$6) but would be detectable in the near future.

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arXiv2009

Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.

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arXiv2009

The GMRT Radio Halo Survey and low frequency follow-up

The GMRT Radio Halo Survey, carried out at 610 MHz to investigate the statistical properties of cluster radio halos in a complete cluster sample selected in the redshift interval z=0.2-0.4, has significantly improved our understanding of the origin of cluster radio halos and relics. Here we briefly summarize the most relevant results of our investigation. A low frequency follow-up is in progress with the GMRT at 325 MHz and 240 MHz on the diffuse sources and candidated found at 610 MHz. We briefly report some preliminary results on these low frequency observations. Cluster radio halos with different radio spectral properties have been unexpectedly found.

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arXiv2009

High energy emission from galaxy clusters and particle acceleration due to MHD turbulence

In the next years the FERMI gamma ray telescope and the Cherenkov telescopes will put very stringent constraints to models of gamma ray emission from galaxy clusters providing crucial information on relativistic particles in the inter-galactic-medium. We derive the broad band non-thermal spectrum of galaxy clusters 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 mergers, and discuss the importance of future gamma ray observations.

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arXiv2008

Gamma ray emission and stochastic particle acceleration in galaxy clusters

FERMI (formely GLAST) will shortly provide crucial information on relativistic particles in galaxy clusters. We discuss non-thermal emission 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 mergers. Diffuse cluster-scale radio emission (Radio Halos) and hard X-rays are produced during massive mergers while gamma ray emission, at some level, is expected to be common in galaxy clusters.

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arXiv2008

A low-frequency radio halo associated with a cluster of galaxies

Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant radio haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at cluster center. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies.

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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

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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