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Issue A&A Volume 366, Number 1, January IV 2001 Page(s) 62 - 67 Section Extragalactic astronomy DOI 10.1051/0004-6361:20000098

A&A 366, 62-67 (2001)
DOI: 10.1051/0004-6361:20000098

Reanalysis of the high energy cutoff of the 1997 Mkn 501 TeV energy spectrum

F. A. Aharonian¹, A. G. Akhperjanian², J. A. Barrio^{3, 4}, K. Bernlöhr¹, O. Bolz¹, H. Börst⁵, H. Bojahr⁶, J. L. Contreras⁴, J. Cortina³, S. Denninghoff³, V. Fonseca⁴, J. C. Gonzalez⁴, N. Götting⁷, G. Heinzelmann⁷, G. Hermann¹, A. Heusler¹, W. Hofmann¹, D. Horns⁷, A. Ibarra⁴, C. Iserlohe⁶, I. Jung¹, R. Kankanyan^{1, 2}, M. Kestel³, J. Kettler¹, A. Kohnle¹, A. Konopelko¹, H. Kornmeyer³, D. Kranich³, H. Krawczynski¹, H. Lampeitl¹, E. Lorenz³, F. Lucarelli⁴, N. Magnussen⁶, O. Mang⁵, H. Meyer⁶, R. Mirzoyan³, A. Moralejo⁴, L. Padilla⁴, M. Panter¹, R. Plaga³, A. Plyasheshnikov¹, J. Prahl⁷, G. Pühlhofer¹, W. Rhode⁶, A. Röhring⁷, G. P. Rowell¹, V. Sahakian², M. Samorski⁵, M. Schilling⁵, F. Schröder⁶, M. Siems⁵, W. Stamm⁵, M. Tluczykont⁷, H. J. Völk¹, C. Wiedner¹ and W. Wittek<sup>3

1</sup>  Max Planck Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany
²  Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
³  Max Planck Institut für Physik, Föhringer Ring 6, 80805 München, Germany
⁴  Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, 28040 Madrid, Spain
⁵  Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstraße 15-19, 24118 Kiel, Germany
⁶  Universität Wuppertal, Fachbereich Physik, Gaußstr.20, 42097 Wuppertal, Germany
⁷  Universität Hamburg, II. Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany

(Received 18 August 2000 / Accepted 24 October 2000)

Abstract
Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flares of Markarian 501 (Mkn 501) are reanalyzed using an algorithm providing improved energy resolution. A resolution of 10% to 12% is obtained by accounting for the variation of the Cherenkov light yield with the height of the shower maximum in the atmosphere. The improved energy resolution is particularly relevant for the study of the high-energy cutoff in the spectrum, which might be caused by interactions with the intergalactic infrared background radiation. The reanalysis presented here confirms the results obtained in the previous analysis, but hints a steeper slope of the spectrum in the region around 20 TeV.

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

Offprint request: W. Hofmann, Werner.Hofmann@mpi-hd.mpg.de

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