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Astronomy and Astrophysics 448 (2006) L19-L23
Evidence for VHE Gamma-ray Emission from the Distant BL Lac PG 1553+113
F. Aharonian, A.G. Akhperjanian, A.R. Bazer-Bachi, M. Beilicke, Wystan Benbow, David Berge, K. Bernlohr, C. Boisson, O. Bolz, V. Borrel, I. Braun, F. Breitling, A.M. Brown, R. Buhler, S. Carrigan, P.M. Chadwick, L.-M. Chounet1, R. Cornils, L. Costamante, B. Degrange1, H.J. Dickinson, A. Djannati-Ataï2, L.O'C. Drury, G. Dubus1, K. Egberts, D. Emmanoulopoulos, P. Espigat2, F. Feinstein3, G. Fontaine1, S. Funk, Y. Gallant3, B. Giebels1, J.F. Glicenstein, P. Goret, C. Hadjichristidis, D. Hauser, M. Hauser, G. Heinzelmann, G. Henri, G. Hermann, J.A. Hinton, W. Hofmann, M. Holleran, Dieter Horns, A. Jacholkowska3, O.C. de Jager, B. Khelifi1, S. Klages, Nu. Komin, A. Konopelko, I.J. Latham, R. Le Gallou, A. Lemiere2, M. Lemoine-Goumard1, T. Lohse, J.M. Martin, O. Martineau-Huynh4, A. Marcowith, C. Masterson, T.J.L. Mccomb, M. De Naurois4, D. Nedbal, S.J. Nolan, A. Noutsos, K.J. Orford, J.L. Osborne, M. Ouchrif4, M. Panter, G. Pelletier, S. Pita2, G. Puhlhofer, M. Punch2, B.C. Raubenheimer, M. Raue, S.M. Rayner, A. Reimer, O. Reimer, J. Ripken, L. Rob, L. Rolland4, G. Rowell, V. Sahakian, L. Sauge, S. Schlenker, R. Schlickeiser, C. Schuster, U. Schwanke, M. Siewert, H. Sol, D. Spangler, R. Steenkamp, C. Stegmann, G. Superina, J.-P. Tavernet4, R. Terrier2, C.G. Theoret2, M. Tluczykont1, C. Van Eldik, G. Vasileiadis3, C. Venter, P. Vincent4, H.J. Volk, S.J. Wagner, M. Ward
HESS Collaboration(s)

The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the H.E.S.S. 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 +/- 1.3_{stat} +/- 1.0_{syst}) x10^{-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 +/- 0.6). 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.
1 :  LLR - Laboratoire Leprince-Ringuet
2 :  APC - UMR 7164 - AstroParticule et Cosmologie
3 :  LPTA - Laboratoire de Physique Théorique et Astroparticules
4 :  LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
Physique/Astrophysique/Cosmologie et astrophysique extra-galactique

Planète et Univers/Astrophysique
Galaxies: active – BL lacertae objects: Individual: PG 1553+113 – Gamma rays: observations
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