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Astron. Astrophys. 327, L5-L8

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Letter to the Editor

Measurement of the flux, spectrum, and variability
of TeV FORMULA-rays from Mkn 501 during a state of high activity

F. Aharonian1, A.G. Akhperjanian2, J.A. Barrio3,4, K. Bernlöhr1, J.J.G. Beteta4, S.M. Bradbury3,8, J.L. Contreras4, J. Cortina4, A. Daum1, T. Deckers5, E. Feigl3, J. Fernandez3,4, V. Fonseca4, A. Fraß1, B. Funk6, J.C. Gonzalez4, V. Haustein7, G. Heinzelmann7, M. Hemberger1, G. Hermann1, M. Heß1, A. Heusler1, W. Hofmann1, I. Holl3, D. Horns7, R. Kankanian1,2, O. Kirstein5, C. Köhler1, A. Konopelko1, H. Kornmayer3, D. Kranich3, H. Krawczynski7, H. Lampeitl1, A. Lindner7, E. Lorenz3, N. Magnussen6, H. Meyer6, R. Mirzoyan3,4,2, H. Möller6, A. Moralejo4, L. Padilla4, M. Panter1, D. Petry3, R. Plaga3, J. Prahl7, C. Prosch3, G. Pühlhofer1, G. Rauterberg5, W. Rhode6, R. Rivero7, A. Röhring7, V. Sahakian2, M. Samorski5, J.A. Sanchez4, D. Schmele7, T. Schmidt6, W. Stamm5, M. Ulrich1, H.J. Völk1, S. Westerhoff6, B. Wiebel-Sooth6, C.A. Wiedner1, and M. Willmer5, and H. Wirth1
(HEGRA Collaboration)

1Max-Planck-Institut für Kernphysik, P.O. Box 103980, D-69029 Heidelberg, Germany
2Yerevan Physics Institute, Yerevan, Armenia
3Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany
4Facultad de Ciencias Fisicas, Universidad Complutense, E-28040 Madrid, Spain
5Universität Kiel, Inst. für Kernphysik, Olshausenstr.40, D-24118 Kiel, Germany
6BUGH Wuppertal, Fachbereich Physik, Gaußstr.20, D-42119 Wuppertal, Germany
7Universität Hamburg, II. Inst. für Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg, Germany
8Now at Department of Physics University of Leeds, Leeds LJ2 9JT, UK

Received 3 June 1997 / Accepted 26 August 1997


Between March 16, 1997 and April 14, 1997, a high flux level of TeV FORMULA-rays was observed from Mkn 501, using the HEGRA stereoscopic system of four imaging Cherenkov telescopes. The flux level varied during this period from about one half up to six times the flux observed from the Crab Nebula. Changes of the detection rate by a factor of up to 4 within 1 day have been observed. The measured differential energy spectrum of the radiation follows a power law from 1 TeV to 10 TeV. The differential spectral index of 2.47 FORMULA is close to that of the Crab Nebula of FORMULA.

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

Send offprint requests to: hermann@eu1.mpi-hd.mpg.de

© European Southern Observatory (ESO) 1997

Online publication: October 14, 1997
Last change: April 6, 1998