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.Comment: 12 pages, Invited Review at the XXI Texas Symposium on Relativistic Astrophysics held on December 9--13 2002, in Florence, Ital

Multifrequency VLA radio observations of the X-ray cavity cluster of galaxies RBS797: evidence of differently oriented jets

We report on the peculiar activity of the radio source located at the center of the cooling flow cluster RBS797 (z=0.35), the first distant cluster in which two pronounced X-ray cavities have been discovered. New multifrequency (1.4, 4.8, and 8.4 GHz) observations obtained with the Very Large Array clearly reveal the presence of radio emission on three different scales showing orientation in different directions, all of which indicates that RBS797 represents a very peculiar case. The lowest resolution images show large-scale radio emission characterized by amorphous morphology and a steep spectrum, extended on a scale of hundreds of kpc. On a scale of tens of kpc, there is evidence of 1.4 GHz radio emission elongated in the northeast-southwest direction exactly towards the holes detected in X-rays. The highest resolution image shows the details of the innermost 4.8 GHz radio jets on a kpc scale; they are remarkably oriented in a direction that is perpendicular to that of the extended structure detected at a lower resolution. We therefore find evidence of a strong interaction between the central radio source and the intra-cluster medium in RBS797. We suggest a scenario in which the 1.4 GHz emission filling the X-ray cavities consists of buoyant bubbles of radio emitting plasma that are created by twin jets in the past and whose expansion has displaced the thermal gas that was formerly in the X-ray holes, whereas the two jets visible at 4.8 GHz are related to the present nuclear activity that has restarted at a different position angle from the original outburst that created the outer radio lobes. The total radio luminosity is ~ 10^42 erg/s, corresponding to a factor of a few thousand times less than the estimated cooling luminosity.Comment: 9 pages, 4 figures, accepted for publication in Astronomy & Astrophysics; replaced with revised version corrected for language editin

Radio Halos in Merging Clusters

AbstractThe number of known diffuse radio sources in clusters of galaxies (halos and relics) has grown in recent years, making it possible to derive statistical considerations on the physical conditions of these sources and on the properties of related clusters. We will discuss the percentage of clusters with a diffuse source and will show evidence that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes