A&A 410, 813-821 (2003)
DOI: 10.1051/0004-6361:20031265

TeV -ray light curve and energy spectrum of Mkn 421 during its 2001 flare as measured with HEGRA CT1

F. Aharonian¹, A. Akhperjanian², M. Beilicke³, K. Bernlöhr¹, H. G. Börst⁴, H. Bojahr⁵, O. Bolz¹, J. A. Coarasa⁶, J. L. Contreras⁷, J. Cortina^{6, 8}, S. Denninghoff⁶, V. Fonseca⁷, M. Girma¹, F. Goebel⁶, N. Götting³, G. Heinzelmann³, G. Hermann¹, A. Heusler¹, W. Hofmann¹, D. Horns¹, I. Jung¹, R. Kankanyan¹, M. Kestel⁶, J. Kettler¹, A. Kohnle¹, A. Konopelko¹, D. Kranich⁶, H. Krawczynski^{1, 9}, H. Lampeitl³, M. López⁷, E. Lorenz⁶, F. Lucarelli⁷, O. Mang⁴, H. Meyer⁵, R. Mirzoyan⁶, A. Moralejo⁷, E. Oña-Wilhelmi⁷, D. Paneque⁶, M. Panter¹, A. Plyasheshnikov^{1, 2}, G. Pühlhofer¹, R. de los Reyes⁷, W. Rhode⁵, J. Ripken³, G. Rowell¹, V. Sahakian², M. Samorski⁴, M. Schilling⁴, T. Schweizer⁸, I. Sevilla⁷, M. Siems⁴, D. Sobczynska¹⁰, W. Stamm⁴, M. Tluczykont³, N. Tonello⁶, V. Vitale⁶, H. J. Völk¹, R. M. Wagner⁶, C. A. Wiedner¹ and W. Wittek<sup>6

1</sup>  Max-Planck-Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany
²  Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia
³  Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
⁴  Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstraße 15-19, 24118 Kiel, Germany
⁵  Universität Wuppertal, Fachbereich Physik, Gaußstr. 20, 42097 Wuppertal, Germany
⁶  Max-Planck-Institut für Physik, Föhringer Ring 6, 80805 München, Germany
⁷  Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, 28040 Madrid, Spain
⁸  Institut de Física d'Altes Energies, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
⁹  Now at Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
¹⁰  Experimental Physics Dept., University of Lódz ul. Pomorska 149/153, 90-236 Lódz, Poland

(Received 12 March 2003 / Accepted 7 August 2003)

Abstract
In the first months of 2001 the AGN Mkn 421 showed highly variable, strong TeV activity at flux levels frequently exceeding 1 Crab. Here we present the light curve and energy spectrum of Mkn 421 as measured with the HEGRA stand alone telescope CT1. Around 30% of the data were taken under moonlight conditions. The spectrum shows a significant exponential energy cutoff at around 3.4 TeV. The results from the dark night- and the moon data are in excellent agreement with each other. A significant spectral shape variation depending on the flux level has been found. The TeV light curve is also found to be highly correlated with the X-ray light curve of the RXTE / ASM satellite, showing no significant time lag larger than 0.2 d. The derived correlation coefficient of 0.83 corresponds to a significance.

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

Offprint request: D. Kranich, J. Cortina, daniel@mppmu.mpg.de;cortina@ifae.es

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