EDP Sciences
Free access
Issue
A&A
Volume 436, Number 2, June III 2005
Page(s) L17 - L20
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500113


A&A 436, L17-L20 (2005)
DOI: 10.1051/0004-6361:200500113

Letter

Discovery of VHE gamma rays from PKS 2005-489

F. Aharonian1, A. G. Akhperjanian2, K.-M. Aye3, A. R. Bazer-Bachi4, M. Beilicke5, W. Benbow1, D. Berge1, P. Berghaus6, K. Bernlöhr1, 7, C. Boisson8, O. Bolz1, I. Braun1, F. Breitling7, A. M. Brown3, J. Bussons Gordo9, P. M. Chadwick3, L.-M. Chounet10, R. Cornils5, L. Costamante1, 11, B. Degrange10, A. Djannati-Ataï6, L. O'C. Drury12, G. Dubus10, D. Emmanoulopoulos13, P. Espigat6, F. Feinstein9, P. Fleury10, G. Fontaine10, Y. Fuchs14, S. Funk1, Y. A. Gallant9, B. Giebels10, S. Gillessen1, J. F. Glicenstein15, P. Goret15, C. Hadjichristidis3, M. Hauser13, G. Heinzelmann5, G. Henri14, G. Hermann1, J. A. Hinton1, W. Hofmann1, M. Holleran16, D. Horns1, O. C. de Jager16, B. Khélifi1, Nu. Komin7, A. Konopelko1, 7, I. J. Latham3, R. Le Gallou3, A. Lemière6, M. Lemoine-Goumard10, N. Leroy10, T. Lohse7, O. Martineau-Huynh17, A. Marcowith4, C. Masterson1, 11, T. J. L. McComb3, M. de Naurois17, S.J. Nolan3, A. Noutsos3, K. J. Orford3, J. L. Osborne3, M. Ouchrif17, 11, M. Panter1, G. Pelletier14, S. Pita6, G. Pühlhofer1, 13, M. Punch6, B. C. Raubenheimer16, M. Raue5, J. Raux17, S. M. Rayner3, I. Redondo10, 11, A. Reimer18, O. Reimer18, J. Ripken5, L. Rob19, L. Rolland17, G. Rowell1, V. Sahakian2, L. Saugé14, S. Schlenker7, R. Schlickeiser18, C. Schuster18, U. Schwanke7, M. Siewert18, H. Sol8, R. Steenkamp20, C. Stegmann7, J.-P. Tavernet17, R. Terrier6, C. G. Théoret6, M. Tluczykont10, 11, G. Vasileiadis9, C. Venter16, P. Vincent17, H. J. Völk1 and S. J. Wagner13

1  Max-Planck-Institut für Kernphysik, Heidelberg, Germany,
    e-mail: Wystan.Benbow@mpi-hd.mpg.de;
2  Yerevan Physics Institute, Armenia;
3  University of Durham, Department of Physics, UK;
4  Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France;
5  Universität Hamburg, Institut für Experimentalphysik, Germany;
6  APC, Paris, France;
7  Institut für Physik, Humboldt-Universität zu Berlin, Germany;
8  LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, France;
9  Groupe d'Astroparticules de Montpellier, IN2P3/CNRS, Université Montpellier II, France;
10  Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France;
11  European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
12  Dublin Institute for Advanced Studies, Ireland;
13  Landessternwarte, Königstuhl, Heidelberg, Germany;
14  Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, France;
15  DAPNIA/DSM/CEA, CE Saclay, Gif-sur-Yvette, France;
16  Unit for Space Physics, North-West University, Potchefstroom, South Africa;
17  Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, France;
18  Institut für Theoretische Physik, Lehrstuhl IV, Ruhr-Universität Bochum, Germany;
19  Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic;
20  University of Namibia, Windhoek, Namibia;

(Received 29 March 2005 / Accepted 24 April 2005)

Abstract
The high-frequency peaked BL Lac PKS 2005-489 (z=0.071) was observed in 2003 and 2004 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected at the 6.7$\sigma$ level in the 2004 observations (24.2 h live time), but not in the 2003 data set (27.3 h live time). PKS 2005-489 is the first blazar independently discovered by HESS to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is ( $6.9\pm1.0_{\rm stat}\pm1.4_{\rm syst}) \times 10^{-12}$ cm-2 s-1, corresponding to ~2.5% of the flux observed from the Crab Nebula. The 99% upper limit on the flux in 2003, I(>200 GeV $) < 5.2 \times 10^{-12}$ cm-2 s-1, is smaller than the flux measured in 2004, suggesting an increased level of activity in 2004. However, the data show no evidence for significant variability on any time scale less than a year. An energy spectrum is measured and is characterized by a very soft power law (photon index of $\Gamma=4.0\pm0.4$).


Key words: galaxies: active -- BL Lacertae objects: individual: PKS 2005-489 -- gamma rays: observations

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