Observations of Mkn 421 in 2004 with HESS at large zenith angles

¹ Max-Planck-Institut für Kernphysik, Heidelberg e-mail: Dieter.Horns@mpi-hd.mpg.de
² Yerevan Physics Institute
³ University of Durham, Department of Physics
⁴ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse
⁵ Universität Hamburg, Institut für Experimentalphysik
⁶ Physique Corpusculaire et Cosmologie, IN2P3/CNRS, Collège de France, Paris
⁷ Institut für Physik, Humboldt-Universität zu Berlin
⁸ LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon
⁹ Groupe d'Astroparticules de Montpellier, IN2P3/CNRS, Université Montpellier II
¹⁰ Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau
¹¹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹² Dublin Institute for Advanced Studies
¹³ Landessternwarte Heidelberg, Königstuhl
¹⁴ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier
¹⁵ Service d'Astrophysique, DAPNIA/DSM/CEA, CE Saclay
¹⁶ Unit for Space Physics, North-West University, Potchefstroom
¹⁷ Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS, Universités Paris VI & VII
¹⁸ Institut für Theoretische Physik, Lehrstuhl IV, Ruhr-Universität Bochum
¹⁹ Institute of Particle and Nuclear Physics, Charles University, Prague
²⁰ University of Namibia, Windhoek

Received: 11 March 2005
Accepted: 14 April 2005

Abstract

Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (HESS) in Namibia. The observations were carried out at zenith angles of 60°–65°, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km² at 10 TeV. Roughly 7000 photons from Mkn 421 were accumulated with an average gamma-ray rate of 8 photons/min. The overall significance of the detection exceeds 100 standard deviations. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal flux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 h. The time averaged energy spectrum is curved and is well described by a power-law with a photon index and an exponential cutoff at TeV and an average integral flux above 2 TeV of 3 Crab flux units. Significant variations of the spectral shape are detected with a spectral hardening as the flux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above ≈300 GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during high flux state observed in April 2004.