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Astron. Astrophys. 350, 757-764 (1999)

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Observations of Mkn 421 during 1997 and 1998 in the energy range above 500 GeV with the HEGRA stereoscopic Cherenkov telescope system

F.A. Aharonian 1, A.G. Akhperjanian 7, M. Andronache 4, 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, N. Götting 4, 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, O. Mang 5, H. Meyer 6, R. Mirzoyan 2, A. Moralejo 3, L. Padilla 3, M. Panter 1, D. Petry ****  2, R. Plaga 2, A. Plyasheshnikov[FORMULA]  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, M. Schilling 5, 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, 69029 Heidelberg, Germany
2 Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
3 Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, 28040 Madrid, Spain
4 Universität Hamburg, II. Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
5 Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstrasse 15-19, 24118 Kiel, Germany
6 Universität Wuppertal, Fachbereich Physik, Gaußstrasse 20, 42097 Wuppertal, Germany
7 Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia

Received 27 April 1999 / Accepted 4 August 1999

Abstract

Since its commissioning in fall 1996, the stereoscopic system of Imaging Atmospheric Cherenkov Telescopes (IACTs) of HEGRA, with an energy flux sensitivity [FORMULA] at 1 TeV of 10-11 [FORMULA] for one hour of observation time, has been used to monitor the BL Lac object Mkn 421 on a regular basis. In this paper, we report detailed temporal and spectral information about the TeV characteristics of Mkn 421 in 1997 and 1998. We study the light curve, the shortest time scales of flux variability, the differential spectra on a diurnal basis for several days with good [FORMULA]-ray statistics, and the time-averaged energy spectrum. Special emphasis is put on presenting the data taken during the world-wide April 1997 multiwavelength campaign.

Key words: galaxies: BL Lacertae objects: individual: Mkn 421 – gamma rays: observations

* Now at Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany
** Now at Enrico Fermi Institute, The University of Chicago, 933 East 56th Street, Chicago, IL 60637, USA
*** On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
**** Now at Universidad Autónoma de Barcelona, Instituto de Física d'Altes Energies, 08193 Bellaterra, Spain
***** Now at LPNHE, Universités Paris VI-VII, 4 place Jussieu, 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: October 14, 1999
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