Detection of TeV gamma-rays from the BL Lac 1ES 1959+650 in its low states and during a major outburst in 2002

A&A 406, L9-L13 (2003)
DOI: 10.1051/0004-6361:20030838

Letter

Detection of TeV gamma-rays from the BL Lac 1ES 1959+650 in its low states and during a major outburst in 2002

F. Aharonian¹, A. Akhperjanian², M. Beilicke³, K. Bernlöhr¹, H.-G. Börst⁴, H. Bojahr⁵, O. Bolz¹, T. Coarasa⁶, J. L. Contreras⁷, J. Cortina⁸, S. Denninghoff⁶, M. V. Fonseca⁷, M. Girma¹, N. Götting³, G. Heinzelmann³, G. Hermann¹, A. Heusler¹, W. Hofmann¹, D. Horns¹, I. Jung¹, R. Kankanyan¹, M. Kestel⁶, A. Kohnle¹, A. Konopelko¹, H. Kornmeyer⁶, D. Kranich⁶, H. Lampeitl³, M. Lopez⁷, E. Lorenz⁶, F. Lucarelli⁷, O. Mang⁴, H. Meyer⁵, R. Mirzoyan⁶, A. Moralejo⁷, E. Ona-Wilhelmi⁷, M. Panter¹, A. Plyasheshnikov^{1, 9}, G. Pühlhofer¹, R. de los Reyes⁷, W. Rhode⁵, J. Ripken³, J. Robrade³, G. Rowell¹, V. Sahakian², M. Samorski⁴, M. Schilling⁴, M. Siems⁴, D. Sobzynska^{6, 10}, W. Stamm⁴, M. Tluczykont³, V. Vitale⁶, H. J. Völk¹, C. A. Wiedner¹ and W. Wittek⁶

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 Now at Institut de Física d'Altes Energies, UAB, Edifici Cn, 08193, Bellaterra (Barcelona), Spain
⁹ On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
¹⁰ Home institute: University Lodz, Poland

(Received 16 May 2003 / Accepted 4 June 2003)

Abstract
TeV $\gamma$ -rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 $\sigma$ at a value of 5.3% of the Crab flux and in May 2002 during strong outbursts with > $23\,\sigma$ at a flux level of up to 2.2 Crab, making 1ES 1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES 1959+650 is located at a redshift of z = 0.047, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES 1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES 1959+650 during the flares can be fitted by a power law with a spectral index of $2.83 \pm 0.14_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ or by a power law with an exponential cut-off at $(4.2^{+0.8}_{-0.6~{\mbox{\tiny stat}}} \pm 0.9_{\mbox{\tiny sys}})$ TeV and a spectral index of $1.83 \pm 0.15_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ . The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of $3.18 \pm 0.17_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ .

Key words: $\gamma$ -rays: observations -- BL Lacertae objects: individual: 1ES 1959+650

Offprint request: N. Götting, Niels.Goetting@desy.de

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