Evidence for VHE γ-ray emission from the distant BL Lac PG 1553+113

Free access article

Issue		A&A Volume 448, Number 2, March III 2006


Page(s)		L19 - L23
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200600010

A&A 448, L19-L23 (2006)
DOI: 10.1051/0004-6361:200600010

Letter

Evidence for VHE $\gamma$ -ray emission from the distant BL Lac PG 1553+113

F. Aharonian¹, A. G. Akhperjanian², A. R. Bazer-Bachi³, M. Beilicke⁴, W. Benbow¹, D. Berge¹, K. Bernlöhr^{1, 5}, C. Boisson⁶, O. Bolz¹, V. Borrel³, I. Braun¹, F. Breitling⁵, A. M. Brown⁷, R. Bühler¹, S. Carrigan¹, P. M. Chadwick⁷, L.-M. Chounet⁸, R. Cornils⁴, L. Costamante^{1, 9}, B. Degrange⁸, H. J. Dickinson⁷, A. Djannati-Ataï¹⁰, L. O'C. Drury¹¹, G. Dubus⁸, K. Egberts¹, D. Emmanoulopoulos¹², P. Espigat¹⁰, F. Feinstein¹³, G. Fontaine⁸, S. Funk¹, Y. A. Gallant¹³, B. Giebels⁸, J. F. Glicenstein¹⁴, P. Goret¹⁴, C. Hadjichristidis⁷, D. Hauser¹, M. Hauser¹², G. Heinzelmann⁴, G. Henri¹⁵, G. Hermann¹, J. A. Hinton^{1, 12}, W. Hofmann¹, M. Holleran¹⁶, D. Horns¹, A. Jacholkowska¹³, O. C. de Jager¹⁶, B. Khélifi^{8, 1}, Nu. Komin⁵, A. Konopelko⁵, I. J. Latham⁷, R. Le Gallou⁷, A. Lemière¹⁰, M. Lemoine-Goumard⁸, T. Lohse⁵, J. M. Martin⁶, O. Martineau-Huynh¹⁷, A. Marcowith³, C. Masterson^{1, 9}, T. J. L. McComb⁷, M. de Naurois¹⁷, D. Nedbal¹⁸, S. J. Nolan⁷, A. Noutsos⁷, K. J. Orford⁷, J. L. Osborne⁷, M. Ouchrif^{17, 9}, M. Panter¹, G. Pelletier¹⁵, S. Pita¹⁰, G. Pühlhofer¹², M. Punch¹⁰, B. C. Raubenheimer¹⁶, M. Raue⁴, S. M. Rayner⁷, A. Reimer¹⁹, O. Reimer¹⁹, J. Ripken⁴, L. Rob¹⁸, L. Rolland¹⁷, G. Rowell¹, V. Sahakian², L. Saugé¹⁵, S. Schlenker⁵, R. Schlickeiser¹⁹, C. Schuster¹⁹, U. Schwanke⁵, M. Siewert¹⁹, H. Sol⁶, D. Spangler⁷, R. Steenkamp²⁰, C. Stegmann⁵, G. Superina⁸, J.-P. Tavernet¹⁷, R. Terrier¹⁰, C. G. Théoret¹⁰, M. Tluczykont^{8, 9}, C. van Eldik¹, G. Vasileiadis¹³, C. Venter¹⁶, P. Vincent¹⁷, H. J. Völk¹, S. J. Wagner¹² and M. Ward⁷

¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany
e-mail: Wystan.Benbow@mpi-hd.mpg.de
² Yerevan Physics Institute, Armenia
³ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France
⁴ Universität Hamburg, Institut für Experimentalphysik, Germany
⁵ Institut für Physik, Humboldt-Universität zu Berlin, Germany
⁶ LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, France
⁷ University of Durham, Department of Physics, UK
⁸ Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, Palaiseau, France
⁹ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁰ APC, UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris), Paris, France
e-mail: santiago.pita@cdf.in2p3.fr
¹¹ Dublin Institute for Advanced Studies, Ireland
¹² Landessternwarte, Königstuhl, Heidelberg, Germany
¹³ Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, France
¹⁴ DAPNIA/DSM/CEA, CE Saclay, Gif-sur-Yvette, France
¹⁵ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, France
¹⁶ Unit for Space Physics, North-West University, Potchefstroom, South Africa
¹⁷ Laboratoire de Physique Nucléaire et de Hautes Energies, IN2P3/CNRS, Universités Paris VI & VII, France
¹⁸ Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic
¹⁹ Institut für Theoretische Physik, Lehrstuhl IV, Ruhr-Universität Bochum, Germany
²⁰ University of Namibia, Windhoek, Namibia

(Received 14 December 2005 / Accepted 19 January 2006)

Abstract
The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the HESS standard analysis, an excess was measured at the 4.0 $\sigma$ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5 $\sigma$ excesses. The observed integral flux above 200 GeV is $(4.8\pm1.3_{\rm stat}\pm1.0_{\rm syst})\times10^{-12}$ cm^-2 s^-1, and shows no evidence for variability. The measured energy spectrum is characterized by a very soft power law (photon index of $\Gamma=4.0\pm0.6$ ). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than z=0.25 and possibly larger than z=0.78. The observed spectrum is interpreted in the context of VHE $\gamma$ -ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of z<0.74.

Key words: galaxies: active -- BL Lacertae objects: individual: PG 1553+113 -- gamma rays: observations

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Letter

Evidence for VHE -ray emission from the distant BL Lac PG 1553+113

Evidence for VHE $\gamma$ -ray emission from the distant BL Lac PG 1553+113