H.E.S.S. observations of PKS 2155-304

¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany e-mail: wystan.benbow@mpi-hd.mpg.de
² Yerevan Physics Institute, Armenia
³ University of Durham, Department of Physics, UK
⁴ Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France
⁵ Universität Hamburg, Institut für Experimentalphysik, Germany
⁶ Physique Corpusculaire et Cosmologie, IN2P3/CNRS, Collège de France, Paris, France
⁷ Institut für Physik, Humboldt-Universität zu Berlin, Germany
⁸ LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, France
⁹ Groupe d'Astroparticules de Montpellier, IN2P3/CNRS, Université Montpellier II, France
¹⁰ Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, France
¹¹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹² Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France
¹³ Dublin Institute for Advanced Studies, Ireland
¹⁴ Service d'Astrophysique, DAPNIA/DSM/CEA, CE Saclay, Gif-sur-Yvette, France
¹⁵ Landessternwarte, Königstuhl, Heidelberg, Germany
¹⁶ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, France
¹⁷ Unit for Space Physics, North-West University, Potchefstroom, South Africa
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: 17 August 2004
Accepted: 27 September 2004

Abstract

The high-frequency peaked BL Lac PKS 2155-304 at redshift has been detected with high significance (~45σ) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June–September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of . An improved per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (~2σ). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.