the literature (see e.g. [@Reimer:2004ac]). Most importantly, energetic arguments suggest that powerful shocks created in cluster-cluster mergers and in the accretion of material onto the deep cluster gravitational potential well are significant sources of relativistic cosmic rays [@Takizawa:2000qk; @shocks; @Keshet:2002sw; @Blasi:2003xs; @Berrington:2002bh; @Miniati:2001ay; @2008arXiv0806.1522S; @Colafrancesco:1998us]. The same shocks can also re-accelerate originally lower-energy particles injected into the ICM through other processes [@Ensslin:1997kw].

The common denominator to the above mentioned scenarios is that Fermi shock acceleration yields a population of non-thermal relativistic cosmic rays. These include primarily high-energy electrons and protons. The former efficiently loose energy by synchrotron emission at radio frequencies as well as through the up-scattering of background radiation to gamma-ray and X-ray frequencies (inverse Compton scattering). Collisions of high-energy cosmic-ray protons with nuclei in the ICM produce, in their hadronic debris, neutral pions promptly decaying into two gamma rays with typical energies and fluxes potentially observable by Fermi-LAT. In addition, the same collisions yield secondary cosmic ray electrons and positrons from the decays of charged pions [@Berrington:2002bh; @Petrosian:2001ph; @Berezinsky:1996wx].

At energies $E_\gamma\gtrsim 0.1$ GeV and for non-merging clusters, or clusters in the intermediate or late merger stages, most of the gamma-ray emission is believed to stem from gamma 