The high-energy \gamma-ray emission of AP Librae

The high-energy -ray emission of AP Librae

H.E.S.S. Collaboration    A. Abramowski Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    F. Aharonian Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland National Academy of Sciences of the Republic of Armenia, Yerevan    F. Ait Benkhali Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    A.G. Akhperjanian Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia National Academy of Sciences of the Republic of Armenia, Yerevan    E. Angüner Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    G. Anton Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    M. Backes University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia    S. Balenderan University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    A. Balzer DESY, D-15738 Zeuthen, Germany Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany    A. Barnacka Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    Y. Becherini Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden,    J. Becker Tjus Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany    K. Bernlöhr Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    E. Birsin Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    E. Bissaldi Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria    J. Biteau Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France now at Santa Cruz Institute for Particle Physics, Department of Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA,    M. Böttcher Centre for Space Research, North-West University, Potchefstroom 2520, South Africa    C. Boisson LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France    J. Bolmont LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    P. Bordas Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    J. Brucker Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    F. Brun Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    P. Brun DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    T. Bulik Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland    S. Carrigan Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    S. Casanova Centre for Space Research, North-West University, Potchefstroom 2520, South Africa Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    P.M. Chadwick University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    R. Chalme-Calvet LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    R.C.G. Chaves DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    A. Cheesebrough University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    M. Chrétien LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    S. Colafrancesco School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050 South Africa    G. Cologna Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    J. Conrad Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-10691 Stockholm, Sweden Wallenberg Academy Fellow,    C. Couturier LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    Y. Cui Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    M. Dalton Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France Funded by contract ERC-StG-259391 from the European Community,    M.K. Daniel University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    I.D. Davids Centre for Space Research, North-West University, Potchefstroom 2520, South Africa University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia    B. Degrange Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France    C. Deil Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    P. deWilt School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia    H.J. Dickinson Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-10691 Stockholm, Sweden    A. Djannati-Ataï 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,    W. Domainko Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    L.O’C. Drury Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland    G. Dubus UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France    K. Dutson Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom    J. Dyks Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    M. Dyrda Instytut Fizyki Ja̧drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland    T. Edwards Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    K. Egberts Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria    P. Eger Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    P. Espigat 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,    C. Farnier Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-10691 Stockholm, Sweden    S. Fegan Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France    F. Feinstein Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    M.V. Fernandes Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    D. Fernandez Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    A. Fiasson Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    G. Fontaine Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France    A. Förster Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    M. Füßling Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany    M. Gajdus Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    Y.A. Gallant Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    T. Garrigoux LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    G. Giavitto DESY, D-15738 Zeuthen, Germany    B. Giebels Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France    J.F. Glicenstein DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    M.-H. Grondin Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    M. Grudzińska Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland    S. Häffner Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    J. Hahn Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    J.  Harris University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    G. Heinzelmann Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    G. Henri UJF-Grenoble 1 / CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France    G. Hermann Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    O. Hervet LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France    A. Hillert Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    J.A. Hinton Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom    W. Hofmann Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    P. Hofverberg Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    M. Holler Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany    D. Horns Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    A. Jacholkowska LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    C. Jahn Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    M. Jamrozy Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland    M. Janiak Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    F. Jankowsky Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    I. Jung Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    M.A. Kastendieck Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    K. Katarzyński Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland    U. Katz Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    S. Kaufmann Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    B. Khélifi 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,    M. Kieffer LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    S. Klepser DESY, D-15738 Zeuthen, Germany    D. Klochkov Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    W. Kluźniak Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    T. Kneiske Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    D. Kolitzus Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria    Nu. Komin Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    K. Kosack DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    S. Krakau Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany    F. Krayzel Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    P.P. Krüger Centre for Space Research, North-West University, Potchefstroom 2520, South Africa Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    H. Laffon Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France    G. Lamanna Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    J. Lefaucheur 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,    A. Lemière 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,    M. Lemoine-Goumard Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France    J.-P. Lenain LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    T. Lohse Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    A. Lopatin Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    C.-C. Lu Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    V. Marandon Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    A. Marcowith Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    R. Marx Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    G. Maurin Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    N. Maxted School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia    M. Mayer Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany    T.J.L. McComb University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    J. Méhault Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France Funded by contract ERC-StG-259391 from the European Community,    P.J. Meintjes Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa,    U. Menzler Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany    M. Meyer Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-10691 Stockholm, Sweden    R. Moderski Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    M. Mohamed Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    E. Moulin DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    T. Murach Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    C.L. Naumann LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    M. de Naurois Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France    J. Niemiec Instytut Fizyki Ja̧drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland    S.J. Nolan University of Durham, Department of Physics, South Road, Durham DH1 3LE, U.K.    L. Oakes Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    H. Odaka Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    S. Ohm Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom    E. de Oña Wilhelmi Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    B. Opitz Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    M. Ostrowski Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland    I. Oya Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    M. Panter Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    R.D. Parsons Max-Planck-Institut für Kernphysik, P.O. 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Box 103980, D 69029 Heidelberg, Germany    G. Pühlhofer Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    M. Punch 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,    A. Quirrenbach Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    S. Raab Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    M. Raue Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    I. Reichardt 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,    A. Reimer Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria    O. Reimer Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria    M. Renaud Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    R. de los Reyes Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    F. Rieger Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    L. Rob Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic    C. Romoli Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland    S. Rosier-Lees Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    G. Rowell School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia    B. Rudak Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    C.B. Rulten LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France    V. Sahakian Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia National Academy of Sciences of the Republic of Armenia, Yerevan    D.A. Sanchez Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    A. Santangelo Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    R. Schlickeiser Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany    F. Schüssler DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    A. Schulz DESY, D-15738 Zeuthen, Germany    U. Schwanke Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    S. Schwarzburg Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany    S. Schwemmer Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    H. Sol LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France    G. Spengler Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany    F. Spies Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    Ł. Stawarz Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland    R. Steenkamp University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia    C. Stegmann Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany DESY, D-15738 Zeuthen, Germany    F. Stinzing Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    K. Stycz DESY, D-15738 Zeuthen, Germany    I. Sushch Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany Centre for Space Research, North-West University, Potchefstroom 2520, South Africa    J.-P. Tavernet LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, F-75252, Paris Cedex 5, France    T. Tavernier 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,    A.M. Taylor Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland    R. Terrier 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,    M. Tluczykont Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany    C. Trichard Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France    K. Valerius Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    C. van Eldik Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    B. van Soelen Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa,    G. Vasileiadis Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France    C. Venter Centre for Space Research, North-West University, Potchefstroom 2520, South Africa    A. Viana Max-Planck-Institut für Kernphysik, P.O. 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Box 103980, D 69029 Heidelberg, Germany    R. White Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom    A. Wierzcholska Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland    P. Willmann Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    A. Wörnlein Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany    D. Wouters DSM/Irfu, CEA Saclay, F-91191 Gif-Sur-Yvette Cedex, France    R. Yang Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany    V. Zabalza Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom    M. Zacharias Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany    A.A. Zdziarski Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland    A. Zech LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France    H.-S. Zechlin and
J. Finke
Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany U.S. Naval Research Laboratory, Code 7653, 4555 Overlook Ave. SW, Washington, DC, 20375-5352
   P. Fortin Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA    D. Horan Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France
Received ; Accepted
Key Words.:
gamma rays: observations – Galaxies : active – Galaxies : jets – BL Lacertae objects: individual objects: AP Librae
offprints: David Sanchez - email : david.sanchez@lapp.in2p3.fr
Pascal Fortin - email : pafortin@cfa.harvard.edu
Jonathan Biteau - email : biteau@in2p3.fr

The -ray spectrum of the low-frequency-peaked BL Lac (LBL) object AP Librae is studied, following the discovery of very-high-energy (VHE; ) -ray emission up to the TeV range by the H.E.S.S. experiment. This makes AP Librae one of the few VHE emitters of the LBL type. The measured spectrum yields a flux of above 130 GeV and a spectral index of . This study also makes use of Fermi-LAT  observations in the high energy (HE, E100 MeV) range, providing the longest continuous light curve (5 years) ever published on this source. The source underwent a flaring event between MJD 56306-56376 in the HE range, with a flux increase of a factor 3.5 in the 14-day bin light curve and no significant variation in spectral shape with respect to the low-flux state. While the H.E.S.S.  and (low state) Fermi-LAT fluxes are in good agreement where they overlap, a spectral curvature between the steep VHE spectrum and the Fermi-LAT  spectrum is observed. The maximum of the -ray emission in the spectral energy distribution is located below the GeV energy range.

1 Introduction

The BL Lac class of blazars constitutes about of both the First (Abdo et al. 2010b; 1LAC) and Second (Ackermann et al. 2011; 2LAC) Fermi  Large Area Telescope (LAT) Catalogue of Active Galactic Nuclei (AGN), and constitutes the majority of the extragalactic very-high-energy (VHE, E100 GeV) -ray sources111An up-to-date VHE -ray catalogue can be found in the TeVCat, http://tevcat.uchicago.edu. AP Librae falls into the category of “low-frequency-peaked BL Lac” (LBL), defined by an X-ray to radio flux ratio of (Padovani & Giommi 1995), and of the more recently introduced “low frequency synchrotron peaked” (LSP) class of blazars defined by a synchrotron emission peak in the spectral energy distribution (SED) at (see Abdo et al. 2010c). This is an order of magnitude lower than the values found in the bulk of VHE -ray emitting blazars, which belong to the “high-frequency-peaked BL Lac/high frequency synchrotron peaked” (HBL/HSP) class. A continuity between these classes of blazars is suggested by the “blazar sequence” (Fossati et al. 1998), where the dominance of the high-energy component and its peak emission energy are inversely proportional to the total luminosity.

AP Librae was amongst the first few objects to be classified as a member of the BL Lac class (Strittmatter et al. 1972), for which a reliable redshift could be measured (; Disney et al. 1974). The initial redshift measurement is consistent with the most recent measurement from the 6dF galaxy survey (; Jones et al. 2009). An object coincident with AP Librae was discovered in the radio band (PKS 1514–24) during a survey made with the 210-ft reflector at Parkes (Bolton et al. 1964), but it was not until 1971 that the optically variable source AP Librae and the radio source PKS 1514–24 were formally associated (Bond 1971; Biraud 1971). The host galaxy harbors a black hole at its center with a mass, estimated using stellar velocity dispersion, of (Woo et al. 2005).

In X-rays, AP Librae was first detected by the Einstein X-Ray Observatory (1E 1514.72411; Schwartz & Ku 1983). At high energies (HE, E100 MeV), the source 3EG J15172538 (Hartman et al. 1999) was tentatively associated with AP Librae. The photon index reported in the third EGRET catalog was rather soft (), resulting in a low extrapolated flux level in the VHE range covered by atmospheric Cherenkov telescopes. Observations with the University of Durham Mark 6 -ray telescope resulted in a flux upper limit of for (Armstrong et al. 1999; Chadwick et al. 1999).

An early catalog of bright -ray sources detected by the Fermi-LAT  was produced using the first three months of data (Abdo et al. 2009a). One of these sources, 0FGL J1517.92423, was associated with AP Librae, but its photon index was harder (, Abdo et al. 2009b) than that reported for 3EG J15172538. The extrapolation of its spectrum to higher energies raised the possibility of a detection by Cherenkov telescopes. In 2010, the H.E.S.S. Collaboration reported the detection of VHE -rays from AP Librae (Hofmann 2010). Following this announcement, Fortin et al. (2010) showed the first radio-to-TeV SED, based on the preliminary analysis of an HE-VHE data set included in the larger one presented here, while Kaufmann et al. (2011) also pointed out the existence of an X-ray jet resolved with Chandra, making Ap Librae the only known TeV BL Lac object with an extended jet in X-rays.

The paper is organized as follows: in Section 2.1, H.E.S.S. observations are presented while the data analysis of 5 years of Fermi-LAT data is discussed in Section 2.2. The variability and broad-band -ray emission of AP Librae are discussed in Section 3.

2 Observations

2.1 H.E.S.S. observations

The High Energy Stereoscopic System (H.E.S.S.), located in the Khomas Highland in Namibia ( S, E), is an array of telescopes (four at the time of the observations studied) that detect the Cherenkov light flashes from air showers. H.E.S.S. observed AP Librae between MJD 55326 (10 May 2010) and MJD 55689 (8 May 2011) for a total of 34 observations of 28 minutes, each passing data-quality selection criteria (described in Aharonian et al. 2006). This yields an exposure of acceptance-corrected live time with a mean zenith angle of . In order to minimize the spectral gap between Fermi-LAT and H.E.S.S., cuts achieving the lowest possible energy threshold were selected. The loose cuts (Aharonian et al. 2006), which require a minimum shower image intensity of 40 photoelectrons in each camera, were applied to the data set to perform the event selection, yielding an average energy threshold of . The Model analysis method (de Naurois & Rolland 2009) was used to analyze the data within a radius disk centered on the radio core position of AP Librae (, , Johnston et al. 1995) and further extract the spectrum and light curve, using the Reflected-Region method (Berge et al. 2007) to estimate the background contamination. With 1133 on-source events, 9042 off-source events and an on-off normalization of , the significance of the 218 -rays  excess is (standard deviations, Li & Ma 1983). In Figure 1, the background (black crosses) and on-source events distributions (solid histogram) are shown as a function of the squared angular distance between the source position and the -ray direction. The H.E.S.S. point-spread function (PSF) was fitted to the on-source events and matches well both the signal and the background for large angular distances.

A point-like source model, convolved with the PSF, has been fitted to the data. The position obtained through this fit is and , compatible within the statistical errors with the location of the AP Librae core away (Johnston et al. 1995). Further morphological studies confirm the absence of source extension within the H.E.S.S. PSF.

The time-averaged photon spectrum for these data is shown in Figure 2. The best fit is a power-law function, within the energy range , with a probability of , given by:

(1)

where is the decorrelation energy. The best-fit parameters are obtained using a forward folding technique (Piron et al. 2001). Spectral points are derived with a similar approach in restricted energy ranges, with a fixed (to the best fit value) power-law index and a free normalization.

Figure 1: Number of on-source candidate -ray events (solid histogram) and normalized off-source events (crosses), as a function of the squared angular distance from the position of AP Librae, compared to a fit of a modeled PSF (dashed line).

This result was cross-checked with a standard Hillas analysis (Aharonian et al. 2006) with the loose cuts, based also on a different calibration chain. It was found to be entirely compatible with the Model analysis and yielding a detection significance of and a photon index of (see also the comparison of both spectra in Figure 2). The upper limit on the flux derived from observations taken with the University of Durham Mark 6 -ray telescope (Chadwick et al. 1999), corresponding to of the Crab Nebula flux at E300 GeV, is also compatible with the H.E.S.S. spectrum since it is well above the flux level measured here.

Figure 2: The differential VHE -ray spectrum and corresponding butterfly of AP Librae as derived with the Model analysis. Uncertainties on the spectral points are given at , i.e. at the 68.3% confidence level, and upper limits are computed at the 99% confidence level. The residuals, which are the difference between the measured and expected number of -rays  in a bin divided by the expected number of -rays, are shown in the lower panel. The light gray points in the upper plot represent the spectrum derived as a cross-check with the Hillas analysis. Errors are statistical only.

The light curve of the integral flux above , averaged over the time between two successive full moons, is shown in Figure 3. A constant function fit to the time series yields a (/ndf = 3.2/3), which indicates that the light curve does not show any significant variability within the observed statistical errors. A 99% confidence level upper limit on the fractional variance (as defined in Vaughan et al. 2003) of is derived (Feldman & Cousins 1998). No variability is found using the Hillas analysis with the different calibration.

2.2 Fermi-LAT observations

The Fermi-LAT, launched on 2008 June 11, is a pair-conversion -ray detector sensitive to photons in the energy range from 20 MeV to more than 300 GeV (Atwood et al. 2009). The data for this analysis were taken from 4 August 2008 to 4 August 2013 (MJD 54682-56508, 5 years) and were analyzed using the standard Fermi analysis software (ScienceTools v9r32p4) available from the Fermi Science Support Center (FSSC)222http://fermi.gsfc.nasa.gov/ssc/. Events with energy between 300 MeV and 300 GeV were selected from the Pass 7 data set. Only events passing the SOURCE class filter and located within a square region of side length centered on AP Librae were selected. Cuts on the zenith angle () and rocking angle () were also applied to the data. The post-launch P7SOURCE_V6 instrument response functions (IRFs) were used in combination with the corresponding Galactic and isotropic diffuse emission models333http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html. The model of the region includes the diffuse components and all sources from the Second Fermi-LAT Catalog (2FGL, Nolan et al. 2012) located within a square region of side centered on AP Librae. The spectral parameters of the sources were left free during the fitting procedure. A power-law correction in energy with free normalization and spectral slope was applied to the Galactic diffuse component. Events were analyzed using the binned maximum likelihood method as implemented in gtlike.

The source underwent a flaring episode of approximately 10 weeks between MJD 56306-56376 (flaring state). We have therefore defined a quiescent state measured during the periods MJD 54682-56305 and MJD 56377-56508.

AP Librae is detected with a high test statistic of TS=2037 (, Mattox et al. 1996) in the quiescent state. The energy spectrum evaluated using this data set is well described by a power-law with a photon index , in good agreement with the 2FGL value, with no significant indication for spectral curvature. The integral flux is , and the most energetic photon within the containment radius of the Fermi-LAT PSF has an energy of 71 GeV. The systematic uncertainties were evaluated using the bracketing IRFs technique (Ackermann et al. 2012).

Replacing the power-law with a log-parabola444The log-parabola model is defined as results in only a marginal improvement in likelihood ( for 1 degree of freedom, or approximately ). With this model, the best fit differential flux is at 5.48 GeV with an index and a curvature parameter .

The Fermi-LAT spectral error contour for the power-law model of AP Librae is presented in Figure 4. Flux values for individual energy bins were calculated independently, assuming a power-law spectral shape. For each energy bin, the spectral indices of all sources modeled in the region of interest were frozen to the best-fit values obtained for the full energy range and gtlike was used to determine the flux. The superimposed vertical error bars show the statistical uncertainties and the quadratic sum of statistical and systematic uncertainties, respectively. The latter were estimated by Ackermann et al. (2012) to be 10% of the effective area at 100 MeV, 5% at 560 MeV and 10% at 10 GeV and above. 95% confidence level upper limits were calculated for energy bins with TS values below 10. For completeness, the result of the log-parabola fit is also shown in Figure 4.

The variability analysis of the LAT data showed a significant flare starting in 2013 January. During the flaring period MJD 56306-56376, the spectrum is well fitted by a power law with a total flux and a spectral index , consistent in shape with the spectrum during the quiescent period (see Figure 4). The peak flux in the two-week bin light curve is . The flaring state is discussed more extensively in Section 3.1. During this period, some observations were performed with the H.E.S.S. array, but the resulting data were too limited to be useful555Less than 1 hour of useful time was recorded during the flare. The limited duration and poor background estimation do not even give a useful limit on the flux..

Figure 3: Light curves derived from the observations described in Section 2 from MJD 55250 to MJD 55700 (corresponding to the H.E.S.S. measurements). The top panel presents the H.E.S.S. integral flux for where the horizontal bars represent the observing duration elapsed between the two successive full moon periods when H.E.S.S. observed the target. The bottom panel gives the Fermi-LAT  flux, with confidence level upper limits for segments where and the horizontal bars show the 14 day Fermi-LAT  integration times. This sample is typical from what was seen throughout the quiescent state.
Figure 4: The -ray SED of AP Librae from Fermi-LAT  (blue circles) and H.E.S.S. (orange squares and butterfly power-law fit). For the quiescent state, the Fermi-LAT  best-fit power-law (blue butterfly) has been extrapolated toward the H.E.S.S. energy range taking EBL absorption into account (dash-dotted line). The Fermi-LAT  log-parabola fit is shown in gray, and its extrapolation taking the EBL absorption into account is shown in light gray. The flare SED as measured by Fermi-LAT  is given by the red butterfly and open squares. The shorter and longer errors bars indicate statistical-only and the quadratic sum of statistical and systematic uncertainties, respectively (cf. text).

3 Discussion

3.1 The flaring state of AP Librae

The Fermi-LAT light curve of the flaring episode above 300 MeV is shown in Figure 5. The peak flux was 3.5 times greater than the averaged flux. The fastest doubling time scale (as defined in Zhang et al. 1999), corresponds to the rising part and has a value of days. The lightcurve has also been fitted with an asymmetric profile666A symmetric Gaussian profile is rejected at a level of 20 with respect to the function used in this work. where the time of the peak is and the rise and decay time are and . is a constant that is also fitted to the data. Fitting this function to the data yields a peak at , of amplitude , above a constant value of compatible with the low-state flux, for a . The rise time and decay time are found to be days and days, respectively. The rise time (or the doubling time scale) is compatible with 0 at a level, indicating a fast process but the lack of statistics prevents a more precise probe of this event by making shorter time bins. Substructures of the flare possibly present on shorter time scale might be hidden (see Saito et al. 2013, in which study complex flare structures were found when probing smaller time scale). An asymmetry in the rise and decay has already been seen in the GeV range for PKS 1502+106 (Abdo et al. 2010a) and in the TeV range during the 2006 flare of PKS 2155304 (Aharonian et al. 2007). The opposite behavior, i.e. a smaller decay timescale, has nevertheless been observed in the TeV range in the radio galaxy M 87 (Abramowski et al. 2012). However the time scale of the event reported in this work is much longer and might be of a different origin (e.g. the onset and decay of large scale structural changes in the jet or possibly a change in accretion parameters).

Figure 5: Flux above 300 MeV of Ap Librae during the flare detected by the Fermi-LAT with 14 days integration time. The dashed gray line is the result of the fit with an asymmetric exponential profile (see text) plus a constant (gray line).

3.2 The LBL AP Librae

The first evidence for VHE -rays from an LBL-class blazar was the detection of BL Lacertae () at the significance level (Albert et al. 2007) corresponding to a flux that of the Crab Nebula. Its steep VHE spectrum () did not connect smoothly with the harder Fermi-LAT spectrum (Abdo et al. 2009c) () established after the measurement in VHE, but given the significant variability of the HE -ray flux of BL Lacertae (see Sokolovsky et al. 2010; Cutini 2011, 2012 and follow-up ATels), it is possible that the source was in a high VHE flux state at the time it was detected. Further evidence for VHE -ray emission from LBL-type objects was found with the detection of S5 0716+714 (Anderhub et al. 2009), a source with a steep VHE spectrum () and a harder HE spectrum (Ackermann et al. 2011) (). It appears that AP Librae has the smallest spectral change in the HE-VHE bands, with . We note however that according to the current classification of extragalactic VHE -ray emitters in the TeVCaT, (following the classification in the 2LAC), AP Librae would currently be the only VHE -ray emitter of the LBL class.

With , AP Librae has a rather soft HE spectrum among the population of 2FGL AGN also emitting in the VHE regime, for which the average photon index is (Sanchez et al. 2013). Only the BL Lac object 1RXS J101015.9-311909, BL Lacertae, W Comae and S5 0716+714 exhibit a spectral index (Nolan et al. 2012). The observed peak high energy emission in the SED of objects with is localized roughly above 10 GeV. This quantity, generally not known before the advent of Fermi, is of paramount importance for emission modeling (Tavecchio et al. 1998). Note that, Abramowski et al. (2012) reanalyzed the Fermi data of 1RXS J101015.9-311909 above 1 GeV and found a hard index of 1.71, which constrained to be around 100 GeV.

In the next subsection, the peak of the -ray emission of Ap Librae is quantified jointly using the data from H.E.S.S. and Fermi-LAT.

3.3 Broad-band gamma-ray emission of AP Librae

To further investigate the HE-VHE spectral feature, the Fermi-LAT  best fit power-law spectrum was extrapolated to energies greater than and corrected for the EBL attenuation using the model of Franceschini et al. (2008). A comparison of this extrapolation with the H.E.S.S. spectrum yields a (probability ). The H.E.S.S. systematic uncertainties were included by shifting the energy by 10%777This value is slightly more conservative than the one derived by Meyer et al. (2010) using HE and VHE Crab Nebula data., which yields an uncertainty of (see Fig. 4). The same comparison based on an extrapolation of the log-parabola spectral hypothesis yields a (i.e. ), which suggests broad band curvature.

To quantify this curvature, the HE and VHE data points (not corrected for EBL) were fitted with power-law and log-parabola models, taking into account the statistical and systematic uncertainties (Fig. 6). In practice, the fit has been done in log-log space with either a first order (power-law) or a second order (log-parabola) polynomial function. The parameters obtained are given in Table 1. The fit of the data with the power-law yields a of (probability of %), while the log-parabola yields a of (probability of %). A likelihood ratio test prefers the latter model at a level of 4.3, which confirms the presence of curvature in the measured HE-VHE spectrum of AP Librae. However, the fitting method used for the broad band HE-VHE data points differs from the methods used within each energy range and has some limitations (i.e. not taking into account correlations between energy bins). A proper method to overcome such limitations would consist of a joint fit of the data, exploiting the response functions of both space-borne and ground-based -ray instruments, which is beyond the scope of this paper.

Model
power-law - 26.6/13
log-parabola 7.9/12
Table 1: Parameters of the first and second degree polynomial functions fit to the HE and VHE data. The functions are of the form and with .

Correcting the VHE data points for EBL attenuation and repeating the same joint fit, the log-parabola model is then preferred at 2.9. In this case the power-law yields a of (probability of %) and the log-parabola a of (probability of %). Scaling up the EBL absorption by thirty percent, as in Abramowski et al. (2013), or using the model of Finke et al. (2010), does not significantly affect the latter results, due to the rather small redshift of the source.

EBL attenuation is unlikely to be the only explanation of the spectral break observed in the data. An intrinsic spectral turnover could be due to factors such as a break in the underlying electron energy distribution, the onset of the Klein-Nishina regime in the inverse-Compton emission process, or the absorption of -rays on the circumnuclear radiation fields (see the discussion on the possibly related phenomenon of GeV breaks observed in the spectra of flat-spectrum radio quasars: e.g., Finke et al. 2008; Ackermann et al. 2010; Tanaka et al. 2011; Aleksić et al. 2011). To elucidate this conundrum would require extensive multi-wavelength modeling of the SED of this complex object888See, e.g., Tavecchio et al. (2010) who noted the modeling difficulties with simple synchrotron self-Compton radiative scenarios already when VHE measurements were not yet available and with a shorter Fermi-LAT exposure than presented here., which is beyond the scope of this Research Note.

The description of the HE-VHE emission of AP Librae by a log-parabola allows of AP Librae to be estimated at  MeV. This value of about 450 MeV is compatible with the low-energy boundary of the Fermi-LAT range and could then be considered as an upper limit. It can be compared to the values of determined by Abdo et al. (2010c) for the objects BL Lacertae, W Comae and S5 0716+714, using jointly Fermi and publicly available VHE spectra (30 MeV, 4100 MeV and 800 MeV, respectively). Such low-energy emission peaks are rather uncommon with respect to the bulk of extragalactic VHE emitters, which tend to have maximum emissions at or above hundreds of GeV. The broad-band emission of AP Librae is also rather peculiar, as discussed by Fortin et al. (2010) and Kaufmann et al. (2013), with an SED dominated by inverse-Compton and an X-ray spectrum that can not be explained by synchrotron emission, and that might originate from the same mechanism as the -ray emission. This is consistent with a high-energy component shifted toward lower energies and a peak location that could be below the Fermi-LAT energy range.

Figure 6: The -ray SED of AP Librae from Fermi-LAT  (blue circles) and H.E.S.S. (orange squares). The green and blue area represent the 68% error contour of the power-law and log-parabola fit to the HE-VHE data.

4 Conclusions

The LBL class of VHE emitting objects proves to be an interesting laboratory to test radiative model scenarios, and perhaps to identify parameters on which the LBL-HBL sequence could depend. At present, only a handful of LBL objects have been detected at VHE (or just this one, depending on the selection criteria), probably due to a bias toward HBL objects in observation strategies and because LSP objects are the smallest subset of all -ray selected BL Lac objects (Shaw et al. 2013). Observations with the H.E.S.S. II telescope, and the advent of the Cherenkov Telescope Array (CTA), which will open the possibility to perform an extragalactic survey (20% of the sky in 100 hours) with a sensitivity approaching one percent of the flux of the Crab Nebula (Dubus et al. 2012), should allow more LBL-type blazars to be detected, and give better insights into the physical processes at work.

Acknowledgements

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the French Ministry for Research, the CNRS-IN2P3 and the Astroparticle Interdisciplinary Programme of the CNRS, the U.K. Particle Physics and Astronomy Research Council (PPARC), the IPNP of the Charles University, the South African Department of Science and Technology and National Research Foundation, and by the University of Namibia. We appreciate the excellent work of the technical support staff in Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris, Saclay, and in Namibia in the construction and operation of the equipment.

The LAT Collaboration acknowledges support from a number of agencies and institutes for both development and the operation of the LAT as well as scientific data analysis. These include NASA and DOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board in Sweden. Additional support from INAF in Italy and CNES in France for science analysis during the operations phase is also gratefully acknowledged.

The authors want to acknowledge the anonymous referee for his/her help that greatly improved the paper.

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