Probing the extent of the non-thermal emission from the Vela X region at TeV energies with H.E.S.S.

¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
² Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
³ Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
⁴ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁵ National Academy of Sciences of the Republic of Armenia, Yerevan, Armenia
⁶ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁷ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
e-mail: bernhard.glueck@physik.uni-erlangen.de
⁸ University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁹ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
¹⁰ CEA Saclay, DSM/Irfu, 91191 Gif-Sur-Yvette Cedex, France
¹¹ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10, rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
¹² Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹³ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹⁴ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁵ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁶ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252, Paris Cedex 5, France
¹⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹⁸ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁹ Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
²⁰ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²¹ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²² Université Bordeaux 1, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
²³ Funded by contract ERC-StG-259391 from the European Community  
²⁴ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
²⁵ School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
²⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
²⁷ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
²⁸ Instytut Fizyki Ja¸drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²⁹ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³⁰ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université de Savoie, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
e-mail: florent.dubois@lapp.in2p3.fr
³¹ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³² Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87-100 Toruń, Poland
³³ Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
³⁴ School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK

Received: 29 June 2012
Accepted: 28 September 2012

Abstract

Context. Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae, and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) γ-ray emission (HESS J0835−455) was discovered using the H.E.S.S. experiment in 2004. The VHE γ-ray emission was found to be coincident with a region of X-ray emission discovered with ROSAT above 1.5 keV (the so-called Vela X cocoon): a filamentary structure extending southwest from the pulsar to the centre of Vela X.

Aims. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties.

Methods. To increase the sensitivity to the faint γ-ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of γ-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of γ-ray sources but with comparable observation conditions.

Results. The γ-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE γ-ray emission from the PWN HESS J0835−455 is statistically significant over a region of radius 1.2° around the position α = 08^h35^m00^s, δ = −45°36′00′′ (J2000). The Vela X region exhibits almost uniform γ-ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index Γ = 1.32 ± 0.06_stat ± 0.12_sys and an exponential cutoff at an energy of (14.0 ± 1.6_stat ± 2.6_sys) TeV. Compared to the previous H.E.S.S. observations of Vela X the new analysis confirms the general spatial overlap of the bulk of the VHE γ-ray emission with the X-ray cocoon, while its extent and morphology appear more consistent with the (more extended) radio emission, contradicting the simple correspondence between VHE γ-ray and X-ray emissions. Morphological and spectral results challenge the interpretation of the origin of γ-ray emission in the GeV and TeV ranges in the framework of current models.