Volume 437, Number 1, July I 2005
|Page(s)||95 - 99|
|Published online||10 June 2005|
Observations of Mkn 421 in 2004 with HESS at large zenith angles
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
Accepted: 14 April 2005
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.
Key words: galaxies: individual: Mkn 421
© ESO, 2005
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