H.E.S.S. discovery of VHE γ-rays from the quasar PKS 1510−089

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
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 DESY, 15735 Zeuthen, Germany
¹⁰ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
¹¹ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹² CEA Saclay, DSM/Irfu, 91191 Gif-Sur-Yvette Cedex, France
¹³ 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,
¹⁴ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁵ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁶ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁷ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁸ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252 Paris Cedex 5, France
¹⁹ Institut fürAstronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁰ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²¹ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
²² School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050 South Africa
²³ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²⁴ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²⁵ Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²⁶ Funded by contract ERC-StG-259391 from the European Community  
²⁷ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
²⁸ School of Chemistry & Physics, University of Adelaide, 5005 Adelaide, Australia
²⁹ UJF-Grenoble 1/CNRS–INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
³⁰ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
³¹ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
³² Institut für Astro- und Teilchenphysik, Leopold–Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³³ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
³⁴ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³⁵ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³⁶ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³⁷ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 21 January 2013
Accepted: 28 April 2013

Abstract

The quasar PKS 1510−089 (z = 0.361) was observed with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands, to search for very high energy (VHE, defined as E ≥ 0.1 TeV) emission. VHE γ-rays were detected with a statistical significance of 9.2 standard deviations in 15.8 h of H.E.S.S. data taken during March and April 2009. A VHE integral flux of I(0.15 TeV < E < 1.0 TeV)= (1.0 ± 0.2_stat ± 0.2_sys) × 10^-11 cm^-2 s^-1 is measured. The best-fit power law to the VHE data has a photon index of Γ = 5.4 ± 0.7_stat ± 0.3_sys. The GeV and optical light curves show pronounced variability during the period of H.E.S.S. observations. However, there is insufficient evidence to claim statistically significant variability in the VHE data. Because of its relatively high redshift, the VHE flux from PKS 1510−089 should suffer considerable attenuation in the intergalactic space due to the extragalactic background light (EBL). Hence, the measured γ-ray spectrum is used to derive upper limits on the opacity due to EBL, which are found to be comparable with the previously derived limits from relatively-nearby BL Lac objects. Unlike typical VHE-detected blazars where the broadband spectrum is dominated by nonthermal radiation at all wavelengths, the quasar PKS 1510−089 has a bright thermal component in the optical to UV frequency band. Among all VHE detected blazars, PKS 1510−089 has the most luminous broad line region. The detection of VHE emission from this quasar indicates a low level of γ − γ absorption on the internal optical to UV photon field.