Astronomy and Astrophysics, volume 560A, 64-64 (2013/12-1)
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. and WALTER R.
Abstract (from CDS):
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. 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. 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 10–11ph/cm2/s at a 95% confidence level in the 1-300GeV 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 5x10–11ph/cm2/s and 2x10–11ph/cm2/s, respectively. Under the assumption that decaying π0, 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.
galaxies: clusters: general - gamma rays: galaxies: clusters - methods: data analysis - astroparticle physics - rays cosmic
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