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

¹ 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 Łódź ul. Pomorska 149/153, 90-236 Łódź, Poland

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

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.