A&A 477, 353-363 (2008)
DOI: 10.1051/0004-6361:20078516

HESS very-high-energy gamma-ray sources without identified counterparts

F. Aharonian^{1, 2}, A. G. Akhperjanian³, U. Barres de Almeida⁴, A. R. Bazer-Bachi⁵, B. Behera⁶, M. Beilicke⁷, W. Benbow¹, K. Bernlöhr^{1, 8}, C. Boisson⁹, O. Bolz¹, V. Borrel⁵, I. Braun¹, E. Brion¹⁰, A. M. Brown⁴, R. Bühler¹, T. Bulik¹¹, I. Büsching¹², T. Boutelier¹³, S. Carrigan¹, P. M. Chadwick⁴, L.-M. Chounet¹⁴, A. C. Clapson¹, G. Coignet¹⁵, R. Cornils⁷, L. Costamante^{1, 16}, M. Dalton⁸, B. Degrange¹⁴, H. J. Dickinson⁴, A. Djannati-Ataï¹⁷, W. Domainko¹, L.O'C. Drury², F. Dubois¹⁵, G. Dubus¹³, J. Dyks¹¹, K. Egberts¹, D. Emmanoulopoulos⁶, P. Espigat¹⁷, C. Farnier¹⁸, F. Feinstein¹⁸, A. Fiasson¹⁸, A. Förster¹, G. Fontaine¹⁴, Seb. Funk⁸, M. Füßling⁸, Y. A. Gallant¹⁸, B. Giebels¹⁴, J. F. Glicenstein¹⁰, B. Glück¹⁹, P. Goret¹⁰, C. Hadjichristidis⁴, D. Hauser¹, M. Hauser⁶, G. Heinzelmann⁷, G. Henri¹³, G. Hermann¹, J. A. Hinton²⁰, A. Hoffmann²¹, W. Hofmann¹, M. Holleran¹², S. Hoppe¹, D. Horns²¹, A. Jacholkowska¹⁸, O. C. de Jager¹², I. Jung¹⁹, K. Katarzynski²², E. Kendziorra²¹, M. Kerschhaggl⁸, B. Khélifi¹⁴, D. Keogh⁴, Nu. Komin¹⁸, K. Kosack¹, G. Lamanna¹⁵, I. J. Latham⁴, A. Lemière¹⁷, M. Lemoine-Goumard¹⁴, J.-P. Lenain⁹, T. Lohse⁸, J. M. Martin⁹, O. Martineau-Huynh²³, A. Marcowith¹⁸, C. Masterson², D. Maurin²³, G. Maurin¹⁷, T. J. L. McComb⁴, R. Moderski¹¹, E. Moulin¹⁰, M. de Naurois²³, D. Nedbal²⁴, S. J. Nolan⁴, S. Ohm¹, J.-P. Olive⁵, E. de Oña Wilhelmi¹⁷, K. J. Orford⁴, J. L. Osborne⁴, M. Ostrowski²⁵, M. Panter¹, G. Pedaletti⁶, G. Pelletier¹³, P.-O. Petrucci¹³, S. Pita¹⁷, G. Pühlhofer⁶, M. Punch¹⁷, S. Ranchon¹⁵, B. C. Raubenheimer¹², M. Raue⁷, S. M. Rayner⁴, M. Renaud¹, J. Ripken⁷, L. Rob²⁴, L. Rolland¹⁰, S. Rosier-Lees¹⁵, G. Rowell²⁶, B. Rudak¹¹, J. Ruppel²⁷, V. Sahakian³, A. Santangelo²¹, R. Schlickeiser²⁷, F. Schöck¹⁹, R. Schröder²⁷, U. Schwanke⁸, S. Schwarzburg²¹, S. Schwemmer⁶, A. Shalchi²⁷, H. Sol⁹, D. Spangler⁴, L. Stawarz²⁵, R. Steenkamp²⁸, C. Stegmann¹⁹, G. Superina¹⁴, P. H. Tam⁶, J.-P. Tavernet²³, R. Terrier¹⁷, C. van Eldik¹, G. Vasileiadis¹⁸, C. Venter¹², J. P. Vialle¹⁵, P. Vincent²³, M. Vivier¹⁰, H. J. Völk¹, F. Volpe¹⁴, S. J. Wagner⁶, M. Ward⁴, A. A. Zdziarski¹¹, and A. Zech<sup>9

1</sup>  Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
    e-mail: Karl.Kosack@mpi-hd.mpg.de
²  Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland
³  Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁴  University of Durham, Department of Physics, South Road, Durham DH1 3LE, UK
⁵  Centre d'Etude Spatiale des Rayonnements, CNRS/UPS, 9 av. du Colonel Roche, BP 4346, 31029 Toulouse Cedex 4, France
⁶  Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
⁷  Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
⁸  Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁹  LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
¹⁰  DAPNIA/DSM/CEA, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
¹¹  Nicolaus Copernicus Astronomical Center, Warsaw, Poland
¹²  Unit for Space Physics, North-West University, Potchefstroom 2520, South Africa
¹³  Laboratoire d'Astrophysique de Grenoble, INSU/CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France
¹⁴  Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
¹⁵  Laboratoire d'Annecy-le-Vieux de Physique des Particules, CNRS/IN2P3, 9 Chemin de Bellevue, BP 110, 74941 Annecy-le-Vieux Cedex, France
¹⁶  European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG
¹⁷  Astroparticule et Cosmologie (APC), CNRS, Universite Paris 7 Denis Diderot, 10, rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France (UMR 7164 (CNRS, Université Paris VII, CEA, Observatoire de Paris))
¹⁸  Laboratoire de Physique Théorique et Astroparticules, CNRS/IN2P3, Université Montpellier II, CC 70, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁹  Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
²⁰  School of Physics & Astronomy, University of Leeds, Leeds LS2 9JT, UK
²¹  Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²²  Torun Centre for Astronomy, Nicolaus Copernicus University, Torun, Poland
²³  LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 Place Jussieu, 75252 Paris Cedex 5, France
²⁴  Institute of Particle and Nuclear Physics, Charles University, V Holesovickach 2, 180 00 Prague 8, Czech Republic
²⁵  Obserwatorium Astronomiczne, Uniwersytet Jagiellonski, Kraków, Poland
²⁶  School of Chemistry & Physics, University of Adelaide, Adelaide 5005, Australia
²⁷  Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
²⁸  University of Namibia, Private Bag 13301, Windhoek, Namibia

(Received 20 August 2007 / Accepted 2 October 2007 )

Abstract
Context.The detection of gamma rays in the very-high-energy (VHE) energy range (100 GeV-100 TeV) provides a direct view of the parent population of ultra-relativistic particles found in astrophysical sources. For this reason, VHE gamma rays are useful for understanding the underlying astrophysical processes in non-thermal sources.
Aims.We investigate unidentified VHE gamma-ray sources that have been discovered with HESS in the most sensitive blind survey of the Galactic plane at VHE energies conducted so far.
Methods.The HESS array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~ in 25 h observation time for a point-source detection), and with its large field of view, is well suited for scan-based observations. The on-going HESS survey of the inner Galaxy has revealed a large number of new VHE sources, and for each we attempt to associate the VHE emission with multi-wavelength data in the radio through X-ray wavebands.
Results. For each of the eight unidentified VHE sources considered here, we present the energy spectra and sky maps of the sources and their environment. The VHE morphology is compared with available multi-wavelength data (mainly radio and X-rays). No plausible counterparts are found.

Key words: gamma rays: observations -- Galaxy: general -- cosmic rays -- surveys

© ESO 2007

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