Detection of extended very-high-energy γ-ray emission towards the young stellar cluster Westerlund 2

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
² Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
³ Centre d'Étude Spatiale des Rayonnements, CNRS/UPS, 9 Av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁴ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany, e-mail: martin.raue@desy.de
⁵ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁶ LUTH, UMR 8102 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
⁷ DAPNIA/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
⁸ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁹ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹⁰ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹¹ Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, 91128 Palaiseau, France
¹² Laboratoire d'Annecy-le-Vieux de Physique des Particules, IN2P3/CNRS, 9 chemin de Bellevue, BP 110, 74941 Annecy-le-Vieux Cedex, France
¹³ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁴ APC, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France, (UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris))
¹⁵ Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
¹⁶ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹⁷ Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁸ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
¹⁹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁰ Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, 4 place Jussieu, 75252 Paris Cedex 5, France
²¹ Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
²² Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²³ University of Namibia, Private Bag 13301, Windhoek, Namibia

Corresponding authors: olr@stanford.edu olr@stanford.edu

Received: 15 December 2006
Accepted: 5 March 2007

Abstract

Aims. Results from γ-ray observations by the HESS telescope array in the direction of the young stellar cluster Westerlund 2 are presented.

Methods. Stereoscopic imaging of Cherenkov light emission of γ-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts.

Results. The detection of a new γ-ray source is reported from HESS observations in 2006. HESS J1023–575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9σ, and shows extension beyond a point-like object within the HESS point-spread function. The differential γ-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux.

Conclusions.The spatial coincidence between HESS J1023–575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires one to look into a variety of potential models to account for the observed very-high-energy (VHE) γ-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission.