Very high energy gamma-ray observations of the galaxy clusters Abell 496 and Abell 85 with HESS

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany e-mail: wilfried.domainko@mpi-hd.mpg.de
² Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
³ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁴ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁵ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁶ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁷ Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis Diderot, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France, (UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris).)
⁸ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁰ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹¹ IRFU/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette, Cedex, France
¹² Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹³ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹⁴ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁵ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252, Paris Cedex 5, France
¹⁶ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁷ Laboratoire d'Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue – BP 110 74941 Annecy-le-Vieux Cedex, France
¹⁸ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31–342 Kraków, Poland
¹⁹ Laboratoire de Physique Théorique et Astroparticules, CNRS/IN2P3, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
²⁰ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
²¹ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
²² Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²³ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87–100 Toruń, Poland
²⁴ Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic e-mail: Dalibor.Nedbal@mpi-hd.mpg.de
²⁵ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
²⁶ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁷ School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
²⁸ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²⁹ University of Namibia, Private Bag 13301, Windhoek, Namibia

Received: 18 November 2008
Accepted: 5 December 2008

Abstract

Aims. The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high-energy (VHE,  GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe.

Methods. HESS, an array of four imaging atmospheric cherenkov telescopes (IACT), was used to observe the targets in the range of VHE gamma rays.

Results. No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high-density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected are investigated. Upper limits are derived for the gamma-ray flux at energies  GeV for Abell 496 and  GeV for Abell 85.

Conclusions. From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large-scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its higher mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about ~10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluster.