A&A 366, 746-751 (2001)
DOI: 10.1051/0004-6361:20000481

Search for a TeV gamma-ray halo of Mkn 501

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 15 August 2000 / Accepted 10 November 2000)

Abstract
For distant extragalactic sources of gamma-rays in the PeV (10¹⁵ eV) energy range, interactions of the gamma rays with intergalactic diffuse radiation fields will initiate a pair cascade. Depending on the magnetic fields in the vicinity of the source, the cascade can either result in an isotropic halo around an initially beamed source, or remain more or less collimated. Data recorded by the HEGRA system of imaging atmospheric Cherenkov telescopes are used to derive limits on the halo flux from the AGN Mrk 501. This is achieved by comparing the angular distribution of TeV gamma-rays during the 1997 burst phase -where direct photons should dominate -with the distribution during the 1998/99 quiescent state, where a steady-state halo contribution should be most pronounced. The results depend on the assumptions concerning the angular distribution of the halo; limits on the halo flux within to from the source range between 0.1% and 1% of the peak burst flux.

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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© ESO 2001

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