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Astron. Astrophys. 320, L5-L8 (1997)

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Letter to the Editor

Detection of [FORMULA] -rays above 1.5 TeV from Mkn 501

S.M. Bradbury 1, T. Deckers 2, D. Petry 1, A. Konopelko 3, F. Aharonian 3, A.G. Akhperjanian 4, J.A. Barrio 1, 5, A.S. Beglarian 4, J.J.G. Beteta 5, J.L. Contreras 5, J. Cortina 5, A. Daum 3, E. Feigl 1, J. Fernandez 1, 5, V. Fonseca 5, A. Fraß 3, B. Funk 6, J.C. Gonzalez 5, V. Haustein 7, G. Heinzelmann 7, M. Hemberger 3, G. Hermann 3, M. Hess 3, A. Heusler 3, I. Holl 1, W. Hofmann 3, D. Horns 7, R. Kankanian 3, 4, O. Kirstein 2, C. Köhler 3, D. Kranich 1, H. Krawczynski 7, H. Kornmayer 1, H. Lampeitl 3, A. Lindner 7, E. Lorenz 1, N. Magnussen 6, H. Meyer 6, R. Mirzoyan 1, 5, 4, H. Möller 6, A. Moralejo 5, L. Padilla 5, M. Panter 3, R. Plaga 1, J. Prahl 7, C. Prosch 1, G. Pühlhofer 3, G. Rauterberg 2, W. Rhode 6, V. Sahakian 4, M. Samorski 2, J.A. Sanchez 5, D. Schmele 7, W. Stamm 2, M. Ulrich 3, H.J. Völk 3, S. Westerhoff 6, B. Wiebel-Sooth 6, C.A. Wiedner 3, M. Willmer 2 and H. Wirth(HEGRA Collaboration) 3

1 Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany
2 Universität Kiel, Inst. für Kernphysik, Olshausenstr.40, D-24118 Kiel, Germany
3 Max-Planck-Institut für Kernphysik, P.O. Box 103980, D-69029 Heidelberg, Germany
4 Yerevan Physics Institute, Yerevan, Armenia
5 Facultad de Ciencias Fisicas, Universidad Complutense, E-28040 Madrid, Spain
6 BUGH Wuppertal, Fachbereich Physik, Gaußstr.20, D-42119 Wuppertal, Germany
7 Universität Hamburg, II. Inst. für Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg, Germany

Received 29 November 1996 / Accepted 14 January 1997

Abstract

A detection of TeV [FORMULA] -rays from Mkn 501 is reported, based on observations made between March and August 1996 with the first HEGRA Cherenkov telescope (CT1). From the image analysis, 351 excess candidate [FORMULA] -ray events are obtained from the 147 h dataset. The statistical significance of the excess is 5.2 [FORMULA]. The average excess rate is [FORMULA] h-1 above the [FORMULA] 1.5 TeV threshold of CT1. Under the assumption that the spectrum of Mkn 501 follows a power law we find a differential spectral index of 2.6 [FORMULA] and obtain a time-averaged integral flux above 1.5 TeV of [FORMULA] cm-2 s-1. Comparison with our near contemporary observations of the Crab Nebula, used as a standard candle to test CT1 after upgrading to a 127 pixel camera, indicates that Mkn 501 has a spectrum similar to that of the Crab Nebula above 1.5 TeV. The integral flux above 1.5 TeV from Mkn 501 is found to have been between 2.2 and 3.6 times smaller than that from the Crab Nebula. HEGRA is the second experiment to have detected Mkn 501 in the TeV range.

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

Send offprint requests to: petry@hegra1.mppmu.mpg.de

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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