Probing the cosmic-ray content of galaxy clusters by stacking Fermi-LAT count maps

¹ Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
e-mail: ben.huber@physik.uzh.ch
² Department of Astronomy, University of Geneva, ch. d’Écogia 16, 1290 Versoix, Switzerland
e-mail: celine.tchernin@unige.ch; dominique.eckert@unige.ch
³ The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, Albanova, 10691 Stockholm, Sweden
e-mail: christian.farnier@fysik.su.se

Received: 23 May 2013
Accepted: 2 October 2013

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 × 10^-11   ph   cm^-2   s^-1 and 2 × 10^-11   ph   cm^-2   s^-1, respectively.

Conclusions. 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.