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

F. Aharonian; A. G. Akhperjanian; A. R. Bazer-Bachi; M. Beilicke; W. Benbow; D. Berge; K. Bernlöhr; C. Boisson; O. Bolz; V. Borrel; I. Braun; F. Breitling; A. M. Brown; R. Bühler; S. Carrigan; P. M. Chadwick; L.-M. Chounet; R. Cornils; L. Costamante; B. Degrange; H. J. Dickinson; A. Djannati-Ataï; L. O'C. Drury; G. Dubus; K. Egberts; D. Emmanoulopoulos; P. Espigat; F. Feinstein; G. Fontaine; S. Funk; Y. A. Gallant; B. Giebels; J. F. Glicenstein; P. Goret; C. Hadjichristidis; D. Hauser; M. Hauser; G. Heinzelmann; G. Henri; G. Hermann; J. A. Hinton; W. Hofmann; M. Holleran; D. Horns; A. Jacholkowska; O. C. de Jager; B. Khélifi; Nu. Komin; A. Konopelko; I. J. Latham; R. Le Gallou; A. Lemière; M. Lemoine-Goumard; T. Lohse; J. M. Martin; O. Martineau-Huynh; A. Marcowith; C. Masterson; T. J. L. McComb; M. de Naurois; D. Nedbal; S. J. Nolan; A. Noutsos; K. J. Orford; J. L. Osborne; M. Ouchrif; M. Panter; G. Pelletier; S. Pita; G. Pühlhofer; M. Punch; B. C. Raubenheimer; M. Raue; S. M. Rayner; A. Reimer; O. Reimer; J. Ripken; L. Rob; L. Rolland; G. Rowell; V. Sahakian; L. Saugé; S. Schlenker; R. Schlickeiser; C. Schuster; U. Schwanke; M. Siewert; H. Sol; D. Spangler; R. Steenkamp; C. Stegmann; G. Superina; J.-P. Tavernet; R. Terrier; C. G. Théoret; M. Tluczykont; C. van Eldik; G. Vasileiadis; C. Venter; P. Vincent; H. J. Völk; S. J. Wagner; M. Ward

doi:10.1051/0004-6361:200600010

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Volume 448 / No 2 (March III 2006)

A&A, 448 2 (2006) L19-L23

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


Page(s)		L19 - L23
Section		Letters
DOI		https://doi.org/10.1051/0004-6361:200600010
Published online		24 February 2006

A&A 448, L19-L23 (2006)

Letter to the Editor

Evidence for VHE γ-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. Bernlöhr¹^,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. Costamante¹^,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. Hinton¹^,12, W. Hofmann¹, M. Holleran¹⁶, D. Horns¹, A. Jacholkowska¹³, O. C. de Jager¹⁶, B. Khélifi⁸^,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. Masterson¹^,9, T. J. L. McComb⁷, M. de Naurois¹⁷, D. Nedbal¹⁸, S. J. Nolan⁷, A. Noutsos⁷, K. J. Orford⁷, J. L. Osborne⁷, M. Ouchrif¹⁷^,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. Tluczykont⁸^,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 4.0σ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5σ 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 γ-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

© ESO, 2006

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