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
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 
.
Key words: galaxies: active / BL Lacertae objects: individual: PG 1553+113 / gamma rays: observations
© ESO, 2006