Energy dependent γ-ray morphology in the pulsar wind nebula HESS J1825–137

¹ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany e-mail: Stefan.Funk@mpi-hd.mpg.de
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
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 University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁸ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
⁹ Laboratoire Leprince-Ringuet, IN2P3/CNRS, École Polytechnique, 91128 Palaiseau, France
¹⁰ European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹¹ APC, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France
¹² Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
¹³ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
¹⁴ Laboratoire de Physique Théorique et Astroparticules, IN2P3/CNRS, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁵ DAPNIA/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette, Cedex, France
¹⁶ Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁸ Laboratoire de Physique Nucléaire et de Hautes Énergies, IN2P3/CNRS, Universités Paris VI & VII, 4 place Jussieu, 75252 Paris Cedex 5, France
¹⁹ Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
²⁰ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²¹ University of Namibia, Private Bag 13301, Windhoek, Namibia
²² Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany

Received: 4 May 2006
Accepted: 17 July 2006

Abstract

Aims.We present results from deep γ-ray observations of the Galactic pulsar wind nebula HESS J1825–137 performed with the HESS array.

Methods. Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) γ-ray aspects of this object with unprecedented precision.

Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The γ-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826–1334, that is thought to power the pulsar wind nebula. The differential γ-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825–137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology.

Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE γ-ray regime. The VHE γ-ray emission of HESS J1825–137 is phenomenologically discussed in the scenario where the γ-rays are produced by VHE electrons via Inverse Compton scattering. The high γ-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past.