Letter to the Editor

Discovery of VHE gamma rays from PKS 2005–489

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'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France;
⁵ Universität Hamburg, Institut für Experimentalphysik, Germany;
⁶ APC, 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 Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France;
¹¹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹² Dublin Institute for Advanced Studies, Ireland;
¹³ Landessternwarte, Königstuhl, Heidelberg, Germany;
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 Énergies, 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: 29 March 2005
Accepted: 24 April 2005

Abstract

The high-frequency peaked BL Lac PKS 2005-489 () was observed in 2003 and 2004 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected at the 6.7σ level in the 2004 observations (24.2 h live time), but not in the 2003 data set (27.3 h live time). PKS 2005-489 is the first blazar independently discovered by HESS to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is ( cm^-2 s^-1, corresponding to ~2.5% of the flux observed from the Crab Nebula. The 99% upper limit on the flux in 2003, I(>200 GeV cm^-2 s^-1, is smaller than the flux measured in 2004, suggesting an increased level of activity in 2004. However, the data show no evidence for significant variability on any time scale less than a year. An energy spectrum is measured and is characterized by a very soft power law (photon index of ).