Testing the cosmic ray content in galaxy clusters

Context. The effective content of cosmic rays (CR) in galaxy clusters remains elusive. The evidence of relativistic electrons (RE) in the subset of clusters endowed with a radio halo remains hardly quantitative in the absence of robust estimates of the magnetic field B(r), derived from Faraday Rotation (FR) measurements. The content in relativistic protons (RP) requires a different approach, the only direct one residing in the detection of their collisional production of gamma rays (GR). Aims. Based on the evidence of merging phenomena in clusters, theory predicts a large content of RP, whose energy density could be a large fraction of the thermal energy. This paper aims to estimate a maximum production of both secondary relativistic electrons, SRE, and GR from the RP that have supposedly accumulated throughout the entire history of a cluster. Methods. SRE and GR production is maximized when the RP and the thermal gas share the same radial profile. The production rate is normalized by adopting a reference value of 0.3 for xi, the ratio of RP to thermal pressure. The SRE content which obtains, when constrained to reproduce the observed radio brightness profile, yields univocally B(r), if the presence of primary RE were negligible. Results. This procedure is applied to four radio-halo clusters (Coma, A2163, A2255, A2319). In these objects, the central value B-0 required is consistent with typical, albeit rather uncertain, values derived from FR, although for A2163 and A2319 no reliable FR estimates are available to strengthen this result. On the other hand, B(r) typically increases beyond the thermal core, a hardly acceptable condition. This problem is alleviated by assuming a mix of SRE and of primary RE (PRE), with the latter becoming the dominant component beyond the thermal core. These results suggest that in clusters without a radio halo detected so far a diffuse radio-emission should also be observable due to SRE alone, and therefore more centrally condensed, provided that. is of the order of 0.3. To encourage deeper radio observations of such clusters, some examples were selected that seem rather promising. Efforts in this direction, if accompanied by FR measurements, could provide highly significant constraints on the CR content in clusters, even before the future GLAST mission will have accomplished the hard task of detecting the GR. A complementary result concerns the excess far UV in the Coma cluster, that some authors have attributed to IC emission from SRE. It is shown that this hypothesis can be excluded, because it requires a RP energy content in excess of the thermal one.