Observations of selected AGN with HESS

¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany; e-mail: wystan.benbow@mpi-hd.mpg.de;
² Yerevan Physics Institute, Armenia;
³ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France;
⁴ Universität Hamburg, Institut für Experimentalphysik, 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, UK;
⁸ Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France;
⁹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁰ APC, Paris, France;
¹¹ Dublin Institute for Advanced Studies, Ireland;
¹² Landessternwarte, Königstuhl, Heidelberg, Germany;
¹³ Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, France;
¹⁴ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, France;
¹⁵ DAPNIA/DSM/CEA, CE Saclay, Gif-sur-Yvette, France;
¹⁶ Unit for Space Physics, North-West University, Potchefstroom, South Africa;
¹⁷ Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS, Universités Paris VI & VII, France;
¹⁸ Institut für Theoretische Physik, Lehrstuhl IV, Ruhr-Universität Bochum, Germany;
¹⁹ Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic;
²⁰ University of Namibia, Windhoek, Namibia;

Received: 19 May 2005
Accepted: 16 June 2005

Abstract

A sample of selected active galactic nuclei (AGN) was observed in 2003 and 2004 with the High Energy Stereoscopic System (HESS), an array of imaging atmospheric-Cherenkov telescopes in Namibia. The redshifts of these candidate very-high-energy (VHE, >100 GeV) γ-ray emitters range from to . Significant detections were already reported for some of these objects, such as PKS 2155-304 and Markarian 421. Marginal evidence (3.1σ) for a signal is found from large-zenith-angle observations of Markarian 501, corresponding to an integral flux of I(>1.65 TeV) = (1.5±0.6±0.3) 10^-12 cm^-2 s^-1 or ~15% of the Crab Nebula flux. Integral flux upper limits for 19 other AGN, based on exposures of ~1 to ~8 h live time, and with average energy thresholds between 160 GeV and 610 GeV, range from 0.4% to 5.1% of the Crab Nebula flux. All the upper limits are the most constraining ever reported for these objects.