Multi-wavelength observations of PKS 2155-304 with HESS

¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany;
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, e-mail: berrie@poly.in2p3.fr;
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 Énergies, IN2P3/CNRS, Universités Paris VI & VII, Paris, France;
¹⁸ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, Germany;
¹⁹ Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic;
²⁰ University of Namibia1, Windhoek, Namibia;

Received: 2 May 2005
Accepted: 22 June 2005

Abstract

The High Energy Stereoscopic System (HESS) has observed the high-frequency peaked BL Lac object PKS 2155-304 in 2003 between October 19 and November 26 in Very High Energy (VHE) γ-rays ( for these observations). Observations were carried out simultaneously with the Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer satellite (RXTE), the Robotic Optical Transient Search Experiment (ROTSE) and the Nançay decimetric radiotelescope (NRT). Intra-night variability is seen in the VHE band, the source being detected with a high significance on each night it was observed. Variability is also found in the X-ray and optical bands on kilosecond timescales, along with flux-dependent spectral changes in the X-rays. A transient X-ray event with a 1500 s timescale is detected, making this the fastest X-ray flare seen in this object. No correlation can be established between the X-ray and the γ-ray fluxes, or any of the other wavebands, over the small range of observed variability. The average HESS spectrum shows a very soft power law shape with a photon index of . The energy outputs in the 2– and in the VHE γ-ray range are found to be similar, with the X-rays and the optical fluxes at a level comparable to some of the lowest historical measurements, indicating that PKS 2155-304 was in a low or quiescent state during the observations. Both a leptonic and a hadronic model are used to derive source parameters from these observations. These parameters are found to be sensitive to the model of Extragalactic Background Light (EBL) that attenuates the VHE signal at this source's redshift ().