Measurement of the extragalactic background light imprint on the spectra of the brightest blazars observed with H.E.S.S.^⋆

¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
² Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ National Academy of Sciences of the Republic of Armenia, Yerevan, Armenia
⁶ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁷ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁸ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁹ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹⁰ CEA Saclay, DSM/Irfu, 91191 Gif-Sur-Yvette Cedex, France
¹¹ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
¹² Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹³ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁴ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁵ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁶ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252 Paris Cedex 5, France
¹⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁸ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁹ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
²⁰ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²¹ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²² Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²³ Funded by contract ERC-StG-259391 from the European Community  
²⁴ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
²⁵ School of Chemistry & Physics, University ofAdelaide, 5005 Adelaide, Australia
²⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble, France
²⁷ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
²⁸ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²⁹ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³⁰ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
³¹ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³² Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³³ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³⁴ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 10 September 2012
Accepted: 30 November 2012

Abstract

The extragalactic background light (EBL) is the diffuse radiation with the second highest energy density in the Universe after the cosmic microwave background. The aim of this study is the measurement of the imprint of the EBL opacity to γ-rays on the spectra of the brightest extragalactic sources detected with the High Energy Stereoscopic System (H.E.S.S.). The originality of the method lies in the joint fit of the EBL optical depth and of the intrinsic spectra of the sources, assuming intrinsic smoothness. Analysis of a total of  ~10⁵γ-ray events enables the detection of an EBL signature at the 8.8σ level and constitutes the first measurement of the EBL optical depth using very-high energy (E > 100 GeV) γ-rays. The EBL flux density is constrained over almost two decades of wavelengths [0.30 μm, 17 μm] and the peak value at 1.4   μm is derived as λF_λ = 15 ± 2_stat ± 3_sys nW m^-2 sr^-1.