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Astron. Astrophys. 349, 11-28 (1999)

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The time averaged TeV energy spectrum of Mkn 501 of the extraordinary 1997 outburst as measured with the stereoscopic Cherenkov telescope system of HEGRA

F.A. Aharonian 1, A.G. Akhperjanian 7, J.A. Barrio 2,3, K. Bernlöhr *  1, H. Bojahr 6, I. Calle 3, J.L. Contreras 3, J. Cortina 3, A. Daum 1, T. Deckers 5, S. Denninghoff 2, V. Fonseca 3, J.C. Gonzalez 3, G. Heinzelmann 4, M. Hemberger 1, G. Hermann **  1, M. Heß 1, A. Heusler 1, W. Hofmann 1, H. Hohl 6, D. Horns 4, A. Ibarra 3, R. Kankanyan 1,7, J. Kettler 1, C. Köhler 1, A. Konopelko ***  1, H. Kornmeyer 2, M. Kestel 2, D. Kranich 2, H. Krawczynski 1, H. Lampeitl 1, A. Lindner 4, E. Lorenz 2, N. Magnussen 6, H. Meyer 6, R. Mirzoyan 2, A. Moralejo 3, L. Padilla 3, M. Panter 1, D. Petry ****  2, R. Plaga 2, A. Plyasheshnikov***  1, J. Prahl 4, G. Pühlhofer 1, G. Rauterberg 5, C. Renault *****  1, W. Rhode 6, A. Röhring 4, V. Sahakian 7, M. Samorski 5, D. Schmele 4, F. Schröder 6, W. Stamm 5, H.J. Völk 1, B. Wiebel-Sooth 6, C. Wiedner 1, M. Willmer 5 and W. Wittek 2

1 Max-Planck-Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany
2 Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany
3 Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, E-28040 Madrid, Spain
4 Universität Hamburg, II. Institut für Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg, Germany
5 Universität Kiel, Institut für Kernphysik, Leibnitzstrasse 15, D-24118 Kiel, Germany
6 Universität Wuppertal, Fachbereich Physik, Gaussstr.20, D-42097 Wuppertal, Germany
7 Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia

Received 16 March 1999 / Accepted 28 June 1999


During the several months of the outburst of Mkn 501 in 1997 the source has been monitored in TeV [FORMULA]-rays with the HEGRA stereoscopic system of imaging atmospheric Cherenkov telescopes. Quite remarkably it turned out that the shapes of the daily [FORMULA]-ray energy spectra remained essentially stable throughout the entire state of high activity despite dramatic flux variations during this period. The derivation of a long term time-averaged energy spectrum, based on more than 38,000 detected TeV photons, is therefore physically meaningful. The unprecedented [FORMULA]-ray statistics combined with the 20% energy resolution of the instrument resulted in the first detection of [FORMULA]-rays from an extragalactic source well beyond 10 TeV, and the first high accuracy measurement of an exponential cutoff in the energy region above 5 TeV deeply into the exponential regime. From 500 GeV to 24 TeV the differential photon spectrum is well approximated by a power-law with an exponential cutoff: [FORMULA], with [FORMULA], [FORMULA], and [FORMULA]. We summarize the methods for the evaluation of the energy spectrum in a broad dynamical range which covers almost two energy decades, and study in detail the principal sources of systematic errors. We also discuss several important astrophysical implications of the observed result concerning the production and absorption mechanisms of [FORMULA]-rays in the emitting jet and the modifications of the initial spectrum of TeV radiation due to its interaction with the diffuse extragalactic background radiation.

Key words: galaxies: BL Lacertae objects: individual: Mkn 501 – gamma rays: observations

* Now at Forschungszentrum Karlsruhe, P.O. Box 3640, D-76021 Karlsruhe, Germany
** Now at Enrico Fermi Institute, The University of Chicago, 933 East 56th Street, Chicago, IL 60637, USA
*** On live from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
**** Now at Universidad Autónoma de Barcelona, Institut de Fisica d'Altes Energies, E-08193 Bellaterra, Spain
***** Now at LPNHE, Universités Paris VI-VII, 4 place Jussieu, F-75252 Paris Cedex 05, France

Send offprint requests to: Henric Krawczynski

Correspondence to: Henric Krawczynski, Tel.: (Germany) +6221 516 471 (Henric.Krawczynski@mpi-hd.mpg.de)

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999