Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps
2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 560, A64- p.Article in journal (Refereed) Published
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.
Place, publisher, year, edition, pages
2013. Vol. 560, A64- p.
galaxies: clusters: general, gamma rays: galaxies: clusters, methods: data analysis, astroparticle physics, cosmic rays
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-100112DOI: 10.1051/0004-6361/201321947ISI: 000328754500064OAI: oai:DiVA.org:su-100112DiVA: diva2:691794