Letter to the Editor

Discovery of VHE γ-rays from the high-frequency-peaked BL Lacertae object RGB J0152+017

¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany
² Dublin Institute for Advanced Studies, Dublin, Ireland
³ Yerevan Physics Institute, Yerevan, Armenia
⁴ University of Durham, Department of Physics, Durham, UK
⁵ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, Toulouse, France
⁶ Landessternwarte, Universität Heidelberg, Heidelberg, Germany
⁷ Universität Hamburg, Institut für Experimentalphysik, Hamburg, Germany
⁸ Institut für Physik, Humboldt-Universität zu Berlin, Berlin, Germany
⁹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, Meudon, France e-mail: jean-philippe.lenain@obspm.fr
¹⁰ IRFU/DSM/CEA, CE Saclay, Gif-sur-Yvette, France
¹¹ Universität Erlangen-Nürnberg, Physikalisches Institut, Erlangen, Germany
¹² Nicolaus Copernicus Astronomical Center, Warsaw, Poland
¹³ Unit for Space Physics, North-West University, Potchefstroom, South Africa
¹⁴ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, Grenoble, France
¹⁵ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, Palaiseau, France
¹⁶ Laboratoire d'Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, Annecy-le-Vieux, France
¹⁷ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁸ Astroparticule et Cosmologie (APC), CNRS, Université Paris 7 Denis Diderot, Paris; UMR 7164 (CNRS, Université Paris 7, CEA, Observatoire de Paris), France
¹⁹ Laboratoire de Physique Théorique et Astroparticules, CNRS/IN2P3, Université Montpellier II, Montpellier, France
²⁰ School of Physics & Astronomy, University of Leeds, Leeds, UK
²¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Tübingen, Germany
²² Toruń Centre for Astronomy, Nicolaus Copernicus University, Toruń, Poland
²³ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, Paris, France
²⁴ Institute of Particle and Nuclear Physics, Charles University, Prague, Czech Republic e-mail: dalibor.nedbal@mpi-hd.mpg.de
²⁵ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, Kraków, Poland
²⁶ School of Chemistry & Physics, University of Adelaide, Adelaide, Australia
²⁷ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, Bochum, Germany
²⁸ University of Namibia, Windhoek, Namibia

Received: 18 February 2008
Accepted: 26 February 2008

Abstract

Aims. The BL Lac object RGB J0152+017 () was predicted to be a very high-energy (VHE; >100 GeV) γ-ray source, due to its high X-ray and radio fluxes. Our aim is to understand the radiative processes by investigating the observed emission and its production mechanism using the High Energy Stereoscopic System (HESS) experiment.

Methods. We report recent observations of the BL Lac source RGB J0152+017 made in late October and November 2007 with the HESS array consisting of four imaging atmospheric Cherenkov telescopes. Contemporaneous observations were made in X-rays by the Swift and RXTE satellites, in the optical band with the ATOM telescope, and in the radio band with the Nançay Radio Telescope.

Results. A signal of 173 γ-ray photons corresponding to a statistical significance of 6.6σ was found in the data. The energy spectrum of the source can be described by a powerlaw with a spectral index of . The integral flux above 300 GeV corresponds to ~2% of the flux of the Crab nebula. The source spectral energy distribution (SED) can be described using a two-component non-thermal synchrotron self-Compton (SSC) leptonic model, except in the optical band, which is dominated by a thermal host galaxy component. The parameters that are found are very close to those found in similar SSC studies in TeV blazars.

Conclusions. RGB J0152+017 is discovered as a source of VHE γ-rays by HESS The location of its synchrotron peak, as derived from the SED in Swift data, allows clear classification as a high-frequency-peaked BL Lac (HBL).