Entering a new era of high‐energy γ‐ray experiments, there is an exciting quest for the first detection of γ‐ray emission from clusters of galaxies. To complement these observational efforts, we use high‐resolution simulations of a broad sample of galaxy clusters, and follow self‐consistent cosmic ray (CR) physics using an improved spectral description. We study CR proton spectra as well as the different contributions of the pion decay and inverse‐Compton emission to the total flux and present spectral index maps. We find a universal spectrum of the CR component in clusters with surprisingly little scatter across our cluster sample. When CR diffusion is neglected, the spatial CR distribution also shows approximate universality; it depends however on the cluster mass. This enables us to derive a semi‐analytic model for both the distribution of CRs as well as the pion decay γ‐ray emission and the secondary radio emission that results from hadronic CR interactions with ambient gas protons. In addition, we provide an analytic framework for the inverse‐Compton emission that is produced by shock‐accelerated CR electrons and is valid in the full γ‐ray energy range. Combining the complete sample of the brightest X‐ray clusters observed by ROSAT with our γ‐ray scaling relations, we identify the brightest clusters for the γ‐ray space telescope Fermi and current imaging air Čerenkov telescopes (IACTs) (MAGIC, HESS, VERITAS). We reproduce the previous result of Pfrommer, but provide somewhat more conservative predictions for the fluxes in the energy regimes of Fermi and IACTs when accounting for the bias of ‘artificial galaxies’ in cosmological simulations. We find that it will be challenging to detect cluster γ‐ray emission with Fermi after the second year but this mission has the potential of constraining interesting values of the shock acceleration efficiency after several years of surveying. Comparing the predicted emission from our semi‐analytic model to that obtained by means of our scaling relations, we find that the γ‐ray scaling relations underpredict, by up to an order of magnitude, the flux from cool‐core clusters.

We study the possibility for detecting gamma-ray emission from galaxy clusters. We consider 1) leptophilic models of dark matter (DM) annihilation that include a Sommerfeld enhancement (SFE), 2) different representative benchmark models of supersymmetric DM, and 3) cosmic ray (CR) induced pion decay. Among all clusters/groups of a flux-limited X-ray sample, we predict Virgo, Fornax and M49 to be the brightest DM sources and find a particularly low CR-induced background for Fornax. For a minimum substructure mass given by the DM free-streaming scale, cluster halos maximize the substructure boost for which we find a factor of 1000. Since regions around the virial radius dominate the annihilation flux of substructures, the resulting surface brightness profiles are almost flat. This makes it very challenging to detect this flux with imaging atmospheric Cherenkov telescopes since their sensitivity drops approximately linearly with radius and they typically have 5 − 10 linear resolution elements across a cluster. Assuming cold dark matter with a substructure mass distribution down to an Earth mass and using extended Fermi upper limits, we rule out the leptophilic models in their present form in 28 clusters, and limit the boost from SFE in M49 and Fornax to be 5. This corresponds to a limit on SFE in the Milky Way of 3 which is too small to account for the increasing positron fraction with energy as seen by PAMELA and challenges the DM interpretation. Alternatively, if SFE is realized in Nature, this would imply a limiting substructure mass of M lim > 10 4 M⊙-a problem for structure formation in most particle physics models. Using individual cluster observations, it will be challenging for Fermi to constrain our selection of DM benchmark models without SFE. The Fermi upper limits are, however, closing in on our predictions for the CR flux using an analytic model based on cosmological hydrodynamical cluster simulations. We limit the CR-to-thermal pressure in nearby bright galaxy clusters of the Fermi sample to 10% and in Norma and Coma to 3%. Thus we will soon start to constrain the underlying CR physics such as shock acceleration efficiencies or CR transport properties.PACS numbers: 95.35.+d, 95.85.Pw, 98.62.Gq, 98.70.Sa

Many bright radio relics in the outskirts of galaxy clusters have low inferred Mach numbers, defying expectations from shock acceleration theory and heliospheric observations that the injection efficiency of relativistic particles plummets at low Mach numbers. With a suite of cosmological simulations, we follow the diffusive shock acceleration as well as radiative and Coulomb cooling of cosmic ray electrons during the assembly of a cluster. We find a substantial population of fossil electrons. When reaccelerated at a shock (through diffusive shock acceleration), they are competitive with direct injection at strong shocks and overwhelmingly dominate by many orders of magnitude at weak shocks, M < ∼ 3, which are the vast majority at the cluster periphery. Their relative importance depends on cooling physics and is robust to the shock acceleration model used. While the abundance of fossils can vary by a factor of ∼ 10, the typical reaccelerated fossil population has radio brightness in excellent agreement with observations. Fossil electrons with 1 < ∼ γ < ∼ 100 (10 < ∼ γ < ∼ 10 4 ) provide the main seeds for reacceleration at strong (weak) shocks; we show that these are well-resolved by our simulation. We construct a simple self-similar analytic model which assumes steady recent injection and cooling. It agrees well with our simulations, allowing rapid estimates and physical insight into the shape of the distribution function. We predict that LOFAR should find many more bright steep-spectrum radio relics, which are inconsistent with direct injection. A failure to take fossil cosmic ray electrons into account will lead to erroneous conclusions about the nature of particle acceleration at weak shocks; they arise from well-understood physical processes and cannot be ignored.

Current theories predict relativistic hadronic particle populations in clusters of galaxies in addition to the already observed relativistic leptons. In these scenarios hadronic interactions give rise to neutral pions which decay into γ rays that are potentially observable with the Large Area Telescope (LAT) on board the Fermi space telescope. We present a joint likelihood analysis searching for spatially extended γ-ray emission at the locations of 50 galaxy clusters in four years of Fermi-LAT data under the assumption of the universal cosmic-ray (CR) model proposed by Pinzke & Pfrommer. We find an excess at a significance of 2.7σ , which upon closer inspection, however, is correlated to individual excess emission toward three galaxy clusters: A400, A1367, and A3112. We discuss these cases in detail and conservatively attribute the emission to unmodeled background systems (for example, radio galaxies within the clusters).Through the combined analysis of 50 clusters, we exclude hadronic injection efficiencies in simple hadronic models above 21% and establish limits on the CR to thermal pressure ratio within the virial radius, R 200 , to be below 1.25%-1.4% depending on the morphological classification. In addition, we derive new limits on the γ-ray flux from individual clusters in our sample.