The origin of cosmic rays in galaxy clusters

Cosmic rays (electrons and protons) are actually expected to be produced or injected in the cluster atmospheres by different mechanisms. X-ray observations and N-body simulations both indicate the existence of shocks in the 

The large size ( Mpc) of radio halos compared to the short paths traveled by high-E electrons - which rapidly loose their energy due to Compton and synchrotron losses (Longair 1993) - require, however, either an efficient and isotropic re-acceleration mechanism or an in-situ, stationary production. 

Two distinct families of models for the CR origin of non-thermal phenomena in galaxy clusters have been proposed so far i) the electronic and the ii) hadronic models. 

Electronic models (first proposed by Jaffe 1977) deal with primarily accelerated electrons and rely on the existence of an efficient and omnipresens re-acceleration mechanism. These models also invoke low values of the volume averaged magnetic held I3IX 0.2 (Fusco-Femiano et al. 2004), require a rather high CR injection to fit the available HXR data, predict consequently a substantial amount of gamma-ray emission which is spatially concentrated, 

Hadronic models deal with secondary electrons (first proposed by Dennison 1980) produced by the decay of collision/annihilation products (mainly 7r=b — e ) of p-p (Colafrancesco Blasi 1998, Blasi Colafirancesco 1999) or x- X (e.g., Colafrancesco Mele 2001) interactions. The secondary electrons are produced in situ (thus avoiding to invoke re-acceleration) and require values 1 — 10, found to be more consistent with Faraday Rotation data. These models predict a substantial p/e ratio (like that observed in our Galaxy and in SNe remnants) and an extended modest gamma-ray emission with both hadronic and electronic signatures. The in-situ character of these models produce an extended emission both in space and in time (it is actually stationary) and a quite strong feedback on the ICM (Colafrancesco 1999, Miniafi et al. 2001). 

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