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

¹ 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 , 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 γ-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 Γ = . H 2356-309 is one of the most distant BL Lac objects detected at very-high-energy γ-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.