Letter to the Editor

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

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  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 ). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than and possibly larger than . 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 .