GeV Gamma-ray Flux Upper Limits from Clusters of Galaxies

The detection of diffuse radio emission associated with clusters of galaxies indicates populations of relativistic leptons infusing the intracluster medium. Those electrons and positrons are either injected into and accelerated directly in the intracluster medium, or produced as secondary pairs by cosmic-ray ions scattering on ambient protons. Radiation mechanisms involving the energetic leptons together with decay of neutral pions produced by hadronic interactions have the potential to produce abundant GeV photons. Here, we report on the search for GeV emission from clusters of galaxies using data collected by the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope (Fermi) from August 2008 to February 2010. Thirty-three galaxy clusters have been selected according to their proximity and high mass, X-ray flux and temperature, and indications of non-thermal activity for this study. We report upper limits on the photon flux in the range 0.2–100 GeV towards a sample of observed clusters (typical values 1–5 ×10−9 ph cm−2 s−1) considering both point-like and spatially resolved models for the high-energy emission, and discuss how these results constrain the characteristics of energetic leptons and hadrons, and magnetic fields in the intracluster medium. The volume-averaged relativistic-hadron-to-thermal energy density ratio is found to be < 5–10 % in several clusters. Subject headings: cosmic rays — galaxies: clusters: general — gamma rays: galaxies: clusters — radiation mechanisms: non-thermal 1 Corresponding authors: K. Bechtol, bechtol@stanford.edu; P. Blasi, blasi@arcetri.astro. it; S. Funk, funk@slac.stanford.edu; V. Petrosian, vahep@stanford.edu; O. Reimer, olaf.reimer@uibk.ac. at. 2 W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA 3 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, I-56127 Pisa, Italy 4 Laboratoire AIM, CEA-IRFU/CNRS/Université Paris Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur Yvette, France 5 Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, I-34127 Trieste, Italy 6 Dipartimento di Fisica, Università di Trieste, I-34127 Trieste, Italy 7 Istituto Nazionale di Fisica Nucleare, Sezione di Padova, I-35131 Padova, Italy 8 Dipartimento di Fisica “G. Galilei", Università di Padova, I-35131 Padova, Italy 9 Osservatorio Astrofisico di Arcetri, 50125 Firenze, Italy 10 Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, I-06123 Perugia, Italy 11 Dipartimento di Fisica, Università degli Studi di Perugia, I-06123 Perugia, Italy 12 Centre d’Étude Spatiale des Rayonnements, CNRS/UPS, BP 44346, F30128 Toulouse Cedex 4, France 13 Department of Physics, Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA 14 Dipartimento di Fisica “M. Merlin" dell’Università e del Politecnico di Bari, I-70126 Bari, Italy 15 Istituto Nazionale di Fisica Nucleare, Sezione di Bari, 70126 Bari, Italy 16 Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, Palaiseau, France 17 Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, 08193 Barcelona, Spain 18 INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, I-20133 Milano, Italy 19 Agenzia Spaziale Italiana (ASI) Science Data Center, I-00044 Frascati (Roma), Italy 20 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 21 Center for Research and Exploration in Space Science and Technology ar X iv :1 00 6. 07 48 v1 [ as tr oph . H E ] 3 J un 2 01 0