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Date and Time of the Query: 2019-08-24 T00:27:29 PDT
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For refcode 2013A&A...560A..64H:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
2013A&A...560A..64H Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps Huber, B.; Tchernin, C.; Eckert, D.; Farnier, C.; Manalaysay, A.; Straumann, U.; Walter, R. Abstract. Aims: Radio observations have shown that galaxy clusters are giant reservoirs of cosmic rays (CR). Although a gamma-ray signal from the cluster volume is expected to arise through interactions of CR protons with the ambient plasma, a confirming observation is still missing. Methods: We searched for a cumulative gamma-ray emission in the direction of galaxy clusters by analysing a collection of stacked Fermi-LAT count maps. Additionally, we investigated possible systematic differences in the emission between cool-core and non-cool-core cluster populations. Results: Making use of a sample of 53 clusters selected from the HIFLUGCS catalog, we do not detect a significant signal from the stacked sample. The upper limit on the average flux per cluster derived for the total stacked sample is at the level of a few 10^-11^ ph cm^-2^ s^-1^ at a 95% confidence level in the 1-300 GeV band, assuming power-law spectra with photon indices 2.0, 2.4, 2.8, and 3.2. Separate stacking of the cool-core and non-cool-core clusters in the sample lead to similar values of around 5 x 10^-11^ ph cm^-2^ s^-1^ and 2 x 10^-11^ ph cm^-2^ s^-1^, respectively. Conclusions: Under the assumption that decaying pi^0^, produced in collisions between CRs and the ambient thermal gas, are responsible for gamma-ray emission, we set upper limits on the average CR content in galaxy clusters. For the entire cluster population, our upper limit on the gamma-ray flux translates into an upper limit on the average CR-to-thermal energy ratio of 4.6% for a photon index of 2.4, although it is possible for individual systems to exceed this limit. Our 95% upper limits are at the level expected from numerical simulations, which most likely suggests that the injection of CR at cosmological shocks is less efficient than previously assumed. Key words: galaxies: clusters: general, gamma rays: galaxies: clusters, methods: data analysis, astroparticle physics, cosmic rays
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