Observations of the Crab nebula with HESS
Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany e-mail: firstname.lastname@example.org
2 Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
3 Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
4 Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
5 Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
6 LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
7 University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
8 Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
9 Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, 91128 Palaiseau, France
10 European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
11 APC, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France
12 Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
13 Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
14 Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
15 DAPNIA/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette, Cedex, France
16 Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
17 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
18 Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, 4 place Jussieu, 75252 Paris Cedex 5, France
19 Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
20 Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
21 University of Namibia, Private Bag 13301, Windhoek, Namibia
Accepted: 5 July 2006
Context.The Crab nebula was observed with the HESS stereoscopic Cherenkov-telescope array between October 2003 and January 2005 for a total of 22.9 h (after data quality selection). This period of time partly overlapped with the commissioning phase of the experiment; observations were made with three operational telescopes in late 2003 and with the complete 4 telescope array in January–February 2004 and October 2004–January 2005.
Aims.Observations of the Crab nebula are discussed and used as an example to detail the flux and spectral analysis procedures of HESS. The results are used to evaluate the systematic uncertainties in HESS flux measurements.
Methods.The Crab nebula data are analysed using standard HESS analysis procedures, which are described in detail. The flux and spectrum of γ-rays from the source are calculated on run-by-run and monthly time-scales, and a correction is applied for long-term variations in the detector sensitivity. Comparisons of the measured flux and spectrum over the observation period, along with the results from a number of different analysis procedures are used to estimate systematic uncertainties in the measurements.
Results.The data, taken at a range of zenith angles between and , show a clear signal with over 7500 excess events. The energy spectrum is found to follow a power law with an exponential cutoff, with photon index ± and cutoff energy between 440 GeV and 40 TeV. The observed integral flux above 1 TeV is . The estimated systematic error on the flux measurement is estimated to be 20%, while the estimated systematic error on the spectral slope is 0.1.
Key words: gamma rays: observations / ISM: individual objects: Crab nebula / ISM: supernova remnants
© ESO, 2006