A&A 455, 461-466 (2006)
DOI: 10.1051/0004-6361:20054732

Discovery of very high energy -ray emission from the BL Lacertae object H 2356-309 with the HESS Cherenkov telescopes

F. Aharonian¹, A. G. Akhperjanian², A. R. Bazer-Bachi³, M. Beilicke⁴, W. Benbow¹, D. Berge¹, K. Bernlöhr^{1, 5}, C. Boisson⁶, O. Bolz¹, V. Borrel³, I. Braun¹, F. Breitling⁵, A. M. Brown⁷, R. Bühler¹, I. Büsching⁸, S. Carrigan¹, P. M. Chadwick⁷, L.-M. Chounet⁹, R. Cornils⁴, L. Costamante^{1, 10}, B. Degrange⁹, H. J. Dickinson⁷, A. Djannati-Ataï¹¹, L. O'C. Drury¹², G. Dubus⁹, K. Egberts¹, D. Emmanoulopoulos¹³, P. Espigat¹¹, F. Feinstein¹⁴, E. Ferrero¹³, G. Fontaine⁹, Seb. Funk⁵, S. Funk¹, Y. A. Gallant¹⁴, B. Giebels⁹, J. F. Glicenstein¹⁵, P. Goret¹⁵, C. Hadjichristidis⁷, D. Hauser¹, M. Hauser¹³, G. Heinzelmann⁴, G. Henri¹⁶, G. Hermann¹, J. A. Hinton^{1, 13}, W. Hofmann¹, M. Holleran⁸, D. Horns¹⁷, A. Jacholkowska¹⁴, O. C. de Jager⁸, B. Khélifi^{9, 1}, Nu. Komin⁵, A. Konopelko⁵, I. J. Latham⁷, R. Le Gallou⁷, A. Lemière¹¹, M. Lemoine-Goumard⁹, T. Lohse⁵, J. M. Martin⁶, O. Martineau-Huynh¹⁸, A. Marcowith³, C. Masterson^{1, 10}, T. J. L. McComb⁷, M. de Naurois¹⁸, D. Nedbal¹⁹, S. J. Nolan⁷, A. Noutsos⁷, K. J. Orford⁷, J. L. Osborne⁷, M. Ouchrif^{18, 10}, M. Panter¹, G. Pelletier¹⁶, S. Pita¹¹, G. Pühlhofer¹³, M. Punch¹¹, B. C. Raubenheimer⁸, M. Raue⁴, S. M. Rayner⁷, A. Reimer²⁰, O. Reimer²⁰, J. Ripken⁴, L. Rob¹⁹, L. Rolland¹⁵, G. Rowell¹, V. Sahakian², L. Saugé¹⁶, S. Schlenker⁵, R. Schlickeiser²⁰, U. Schwanke⁵, H. Sol⁶, D. Spangler⁷, F. Spanier²⁰, R. Steenkamp²¹, C. Stegmann⁵, G. Superina⁹, J.-P. Tavernet¹⁸, R. Terrier¹¹, C. G. Théoret¹¹, M. Tluczykont^{9, 10, 22}, C. van Eldik¹, G. Vasileiadis¹⁴, C. Venter⁸, P. Vincent¹⁸, H. J. Völk¹, S. J. Wagner¹³ and M. Ward<sup>7

1</sup>  Max-Planck-Institut für Kernphysik, Heidelberg, Germany
²  Yerevan Physics Institute, Yerevan, Armenia
³  Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France
⁴  Universität Hamburg, Institut für Experimentalphysik, Hamburg, Germany
⁵  Institut für Physik, Humboldt-Universität zu Berlin, Germany
⁶  LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, France
⁷  University of Durham, Department of Physics, Durham, UK
⁸  Unit for Space Physics, North-West University, Potchefstroom, South Africa
⁹  Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France
    e-mail: martin.tluczykont@poly.in2p3.fr
¹⁰  European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹¹  APC, Paris, France ()
¹²  Dublin Institute for Advanced Studies, Ireland
¹³  Landessternwarte, Universität Heidelberg, Germany
¹⁴  Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, France
¹⁵  DAPNIA/DSM/CEA, CE Saclay, France
¹⁶  Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, France
¹⁷  Institut für Astronomie und Astrophysik, Universität Tübingen, Germany
¹⁸  Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, France
¹⁹  Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic
²⁰  Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, Germany
²¹  University of Namibia, Windhoek, Namibia
²²  Now at DESY, Platanenallee 6, 15738 Zeuthen, Germany

(Received 20 December 2005 / Accepted 7 May 2006)

Abstract
The extreme synchrotron BL Lac object H 2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of 40 h live-time with the HESS (High Energy Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 -rays (10 standard deviations above background) from H 2356-309, corresponding to an observed integral flux above 200 GeV of I(>200 GeV) = (4.1 0.5) 10^-12 cm^-2 s^-1 (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N₀ = (4.1 0.5) 10^-13 cm^-2 s^-1 TeV^-1 and a photon index of = . H 2356-309 is one of the most distant BL Lac objects detected at very-high-energy -rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self-Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.

Key words: gamma rays: observations -- galaxies: active -- galaxies: BL Lacertae objects: individual: H 2356-309

© ESO 2006

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