arXiv subjects

Luigina Feretti

Publications and source records attributed to Luigina Feretti.

46 records · Page 3Linked to original sources

arXiv2004

Properties of Cluster Radio Emission

Relevant studies of the non-thermal components of the intracluster medium are performed at radio wavelengths. A number of clusters, indeed, exhibits cluster-wide diffuse radio emission, which is indication of the existence of large scale magnetic fields and of relativistic electrons in the cluster volume. There is strong evidence that the presence of diffuse radio emission is related to cluster merger processes. The details of the halo-merger connection are discussed and a brief outline of current models of halo formation is presented.

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arXiv2003

Confirmation of non-thermal hard X-ray excess in the Coma cluster from two epoch observations

We report the hard X-ray spectrum of the Coma cluster obtained using the PDS data of two independent BeppoSAX observations performed with a time interval of about three years. In both the spectra a non thermal excess with respect to the thermal emission is present at a confidence level of ~ 3.4 sigma. The combined spectrum obtained by adding up the two spectra allows a measurement of the excess at the level of ~ 4.8 sigma at energies above 20 keV. The analysis of the full BeppoSAX data set provides a revised non-thermal X-ray flux which is slightly lower than that previously estimated (Fusco-Femiano et al. 1999) and in agreement with that measured by two RXTE observations. The analysis of the offset fields in our Coma observations provides a possible flux determination of the BL Lac object 1ES 1255+244.

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arXiv2003

Particle acceleration in cooling flow clusters of galaxies: the case of Abell 2626

It has recently been proposed a theoretical model which accounts for the origin of radio mini-halos observed in some cooling flow clusters as related to electron re-acceleration by MHD turbulence (Gitti, Brunetti & Setti 2002). The MHD turbulence is assumed to be frozen into the flow of the thermal ICM and thus amplified in the cooling flow region. Here we present the application of this model to a new mini-halo candidate, the cluster A2626, and compare the results with those obtained for the mini-halo in the Perseus cluster. We present VLA data at 330 MHz and 1.5 GHz of the diffuse radio emission observed in A2626, and we show that its main properties can be explained by the model. We find that the power necessary for the re-acceleration of the relic electron population is only a factor ~ 0.7% of the maximum power that can be extracted by the cooling flow (as estimated on the basis of the standard model). We also discuss the observational properties of known mini-halos in connection with those of host clusters, showing that the radio power of mini--halos increases with the maximum power of cooling flows. This trend is expected in the framework of the model. Possible effects of new Chandra and XMM-Newton estimates of $\dot{M}$ on this trend are considered: we conclude that even if earlier derived cooling rates were overestimated, cooling flow powers are still well above the radio powers emitted by mini-halos.

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arXiv2003

The inner kiloparsec of the jet in 3C264

We present new multi-frequency EVN, MERLIN and VLA observations of the radio source 3C264, sensitive to linear scales ranging from the parsec to several kiloparsecs. The observations confirm the existence of regions with different properties in the first kiloparsec of the jet. The most remarkable feature is the transition between a well collimated narrow jet at distances from the core below 80 pc, to a conical-shaped wide jet, with an opening angle of 20 degrees. Another change of properties, consisting of an apparent deflection of the jet ridge line and a diminution of the surface brightness, occurs at a distance of 300 pc from the core, coincident with the radius of a ring observed at optical wavelengths. Our observations add new pieces of information on the spectrum of the radio-optical jet of 3C264, with results consistent with a synchrotron emission mechanism and a spectrum break frequency in the infrared. Brightness profiles taken perpendicularly to the jet of 3C264 are consistent with a spine brightened jet at distances below 100 pc from the core, and an edge-brightened jet beyond, which can be interpreted as evidence of a transverse jet velocity structure. Our observations do not allow us to distinguish between the presence of a face--on dust and gas disk at the center of the host galaxy of 3C264, or rather an evacuated bubble. However, the properties of the jet structure, the changes in the polarization angle, and the plausible jet orientation can be naturally brought into agreement in the bubble scenario.

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arXiv2003

Cooling Flows and Radio Mini-Halos in Clusters of Galaxies

We apply to radio mini-halo candidates a model for electron re-acceleration in cooling flows (Gitti, Brunetti & Setti 2002). In agreement with the basic idea of the model, we find that the power required for the re-acceleration of the electron population is only a small fraction of the maximum power that can be extracted from the cooling flow (as estimated on the basis of the standard model). Observationally, we notice that the strongest radio mini-halos are found in association with the most powerful cooling flows, and that cooling flow powers are orders of magnitude larger than the integrated radio power. Possible effects of new Chandra and XMM-Newton estimates of $\dot{M}$ on this trend are considered: we conclude that even if earlier derived cooling rates were overestimated, cooling flow powers are still well above the radio powers emitted by mini-halos.

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arXiv2003

Non-thermal phenomena in galaxy clusters

The recent evidences of the presence of magnetic fields and relativistic particles in galaxy clusters are reviewed. The existence of microG-level magnetic fields in cluster atmospheres appears well established from the detection of diffuse radio emission and from studies of Rotation Measure. The fact that the diffuse radio emission is not common in clusters favours the hypothesis that the population of relativistic particles, produced during the cluster formation by AGNs or star formation, are reaccelerated by recent cluster merger processes.

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arXiv2003

Clusters of galaxies in radio

Recent results on the radio emission from galaxy clusters are reviewed, with emphasis both on the radio galaxies and on the diffuse radio emission from the intracluster medium. We show that the formation of the tailed morphology in cluster radio galaxies is due to the effect of the cluster gaseous environment and of its large scale motions in merging clusters. From Chandra X-ray data, it has also become evident that the gas in the hot cluster atmospheres may be displaced by the relativistic plasma. The statistical characteristics of radio galaxies are briefly discussed, and are shown to be surprisingly similar for sources both inside and outside rich clusters. The study of diffuse radio sources originating from the intracluster medium (halos and relics) has presently become a major area of investigation. Halos and relics are detected in massive X-ray luminous clusters which are undergoing violent merger processes. The power of radio halos is correlated with the cluster X-ray luminosity and mass.

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arXiv2002

Modeling radio mini-halos in cooling flow clusters

We have developed a model in which the diffuse synchrotron emission from radio mini-halos, observed in some cooling flow clusters, is due to a relic population of relativistic electrons reaccelerated by MHD turbulence via Fermi-like processes. In this model the energetics is supplied by the cooling flow itself. Here, the model (successfully applied to the Perseus cluster, A426) is preliminarily applied to the possible mini-halo candidate A2626, for which we present VLA data.

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arXiv2000

Observational Properties of Diffuse Halos in Clusters

The number of known diffuse radio sources in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. This is indication of a link between the diffuse relativistic and thermal plasma in clusters of galaxies.

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arXiv2000

Radio Halos and Relics in Clusters of Galaxies and Detection Statistics

New sensitive VLA observations confirm the existence of halo and relic sources in 6 Abell clusters where a diffuse emission was found in the NVSS. We find evidence that the frequency of clusters with halos and relics is larger in clusters with high X-ray luminosity. The evidence that the occurrence of a halo source is larger at high redshifts is marginal.

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