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. Chounet (1) , R. Cornils , L. Costamante , B. Degrange (1) , H.J. Dickinson , A. Djannati-Ataï (2) , L.O'C. Drury , G. Dubus (1) , K. Egberts , D. Emmanoulopoulos , P. Espigat (2) , F. Feinstein (3) , G. Fontaine (1) , S. Funk , Y. Gallant (3) , B. Giebels (1) , 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. Jacholkowska (3) , O.C. de Jager , B. Khelifi (1) , S. Klages , Nu. Komin , A. Konopelko , I.J. Latham , R. Le Gallou , A. Lemiere (2) , M. Lemoine-Goumard (1) , T. Lohse , J.M. Martin , O. Martineau-Huynh (4) , A. Marcowith , C. Masterson , T.J.L. Mccomb , M. de Naurois (4) , D. Nedbal , S.J. Nolan , A. Noutsos , K.J. Orford , J.L. Osborne , M. Ouchrif (4) , M. Panter , G. Pelletier , S. Pita (2) , G. Puhlhofer , M. Punch (2) , B.C. Raubenheimer , M. Raue , S.M. Rayner , A. Reimer , O. Reimer , J. Ripken , L. Rob , L. Rolland (4) , 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. Tavernet (4) , R. Terrier (2) , C.G. Theoret (2) , M. Tluczykont (1) , C. van Eldik , G. Vasileiadis (3) , C. Venter , P. Vincent (4) , H.J. Volk , S.J. Wagner , M. Ward

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

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.

Mots clés: Galaxies: active; BL lacertae objects: Individual: PG 1553+113; Gamma rays: observations

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

F. Aharonian, A.G. Akhperjanian, A.R. Bazer-Bachi, M. Beilicke, Wystan Benbow, et al.. Evidence for VHE Gamma-ray Emission from the Distant BL Lac PG 1553+113. Astronomy and Astrophysics - A&A, 2006, 448, pp.L19-L23. ⟨10.1051/0004-6361:200600010⟩. ⟨in2p3-00025548⟩