Discovery of very high energy γ-ray emission from the BL Lacertae object PKS 0301−243 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, 2 Dublin, 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 Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁸ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁹ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
¹⁰ DESY, 15735 Zeuthen, Germany
¹¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
¹² Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹³ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹⁴ 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
¹⁵ Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁶ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁷ Institut fürAstro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
¹⁸ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹⁹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
²⁰ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252, Paris Cedex 5, France
²¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²² DSM/Irfu, CEA Saclay, 91191 Gif-Sur-Yvette Cedex, France
²³ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁴ now at Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²⁵ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, 2050 Johannesburg, South Africa
²⁶ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²⁷ Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²⁸ Funded by contract ERC-StG-259391 from the European Community  
²⁹ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
³⁰ School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
³¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, 38041, France
³² Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
³³ Instytut Fizyki Ja¸drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
³⁴ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
³⁵ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³⁶ Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³⁷ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³⁸ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 4 April 2013
Accepted: 10 September 2013

Abstract

The active galactic nucleus PKS 0301−243 (z = 0.266) is a high-synchrotron-peaked BL Lac object that is detected at high energies (HE, 100 MeV <E < 100 GeV) by Fermi/LAT. This paper reports on the discovery of PKS 0301−243 at very high energies (E > 100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) from observations between September 2009 and December 2011 for a total live time of 34.9 h. Gamma rays above 200 GeV are detected at a significance of 9.4σ. A hint of variability at the 2.5σ level is found. An integral flux I(E > 200 GeV) = (3.3 ± 1.1_stat ± 0.7_syst) × 10^-12 ph cm^-2 s^-1 and a photon index Γ = 4.6 ± 0.7_stat ± 0.2_syst are measured. Multi-wavelength light curves in HE, X-ray and optical bands show strong variability, and a minimal variability timescale of eight days is estimated from the optical light curve. A single-zone leptonic synchrotron self-Compton scenario satisfactorily reproduces the multi-wavelength data. In this model, the emitting region is out of equipartition and the jet is particle dominated. Because of its high redshift compared to other sources observed at TeV energies, the very high energy emission from PKS 0301−243 is attenuated by the extragalactic background light (EBL) and the measured spectrum is used to derive an upper limit on the opacity of the EBL.