Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field

F. Aharonian*, A. G. Akhperjanian, A. R. Bazer-Bachi, B. Behera, M. Beilicke, W. Benbow, D. Berge, K. Bernlöhr, C. Boisson, O. Bolz, V. Borrel, I. Braun, E. Brion, A. M. Brown, R. Bühler, T. Bulik, I. Büsching, T. Boutelier, S. Carrigan, P. M. ChadwickL. M. Chounet, A. C. Clapson, G. Coignet, R. Cornils, L. Costamante, B. Degrange, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O.C. Drury, G. Dubus, J. Dyks, K. Egberts, D. Emmanoulopoulos, P. Espigat, C. Farnier, F. Feinstein, A. Fiasson, A. Förster, G. Fontaine, Y. Fukui, Seb Funk, S. Funk, M. Füßling, Y. A. Gallant, B. Giebels, J. F. Glicenstein, B. Glück, P. Goret, C. Hadjichristidis, D. Hauser, M. Hauser, G. Heinzelmann, G. Henri, G. Hermann, J. A. Hinton, A. Hoffmann, W. Hofmann, M. Holleran, S. Hoppe, D. Horns, A. Jacholkowska, O. C. De Jager, E. Kendziorra, M. Kerschhaggl, B. Khélifi, Nu Komin, K. Kosack, G. Lamanna, I. J. Latham, R. Le Gallou, A. Lemière, M. Lemoine-Goumard, J. P. Lenain, T. Lohse, J. M. Martin, O. Martineau-Huynh, A. Marcowith, C. Masterson, G. Maurin, T. J.L. McComb, R. Moderski, Y. Moriguchi, E. Moulin, M. De Naurois, D. Nedbal, S. J. Nolan, J. P. Olive, K. J. Orford, J. L. Osborne, M. Ostrowski, M. Panter, G. Pedaletti, G. Pelletier, P. O. Petrucci, S. Pita, G. Pühlhofer, M. Punch, S. Ranchon, B. C. Raubenheimer, M. Raue, S. M. Rayner, O. Reimer, M. Renaud, J. Ripken, L. Rob, L. Rolland, S. Rosier-Lees, G. Rowell, B. Rudak, J. Ruppel, V. Sahakian, A. Santangelo, L. Saugé, S. Schlenker, R. Schlickeiser, R. Schroder, U. Schwanke, S. Schwarzburg, S. Schwemmer, A. Shalchi, H. Sol, D. Spangler, Stawarz, R. Steenkamp, C. Stegmann, G. Superina, T. Takeuchi, P. H. Tam, J. P. Tavernet, R. Terrier, C. Van Eldik, G. Vasileiadis, C. Venter, J. P. Vialle, P. Vincent, M. Vivier, H. J. Völk, F. Volpe, S. J. Wagner, M. Ward

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

217 Scopus citations


Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV -ray band, coupled with those at millimetre radio wavelengths, are motivated by the search for cosmic-ray accelerators in our Galaxy. The old-age mixed-morphology SNR W 28 (distance ∼2 kpc) is a prime target due to its interaction with molecular clouds along its northeastern boundary and other clouds situated nearby.Methods. We observed the W 28 field (for ∼40 h) at very high energy (VHE) -ray energies ( 0.1$]]> TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET 100$]]> MeV data was also undertaken. Results from the NANTEN 4 m telescope Galactic plane survey and other CO observations were used to study molecular clouds. Results. We have discovered VHE -ray emission (HESS J1801-233) coincident with the northeastern boundary of W 28 and a complex of sources (HESS J1800-240A, B and C) ∼0.5° south of W 28 in the Galactic disc. The EGRET source (GRO J1801-2320) is centred on HESS J1801-233 but may also be related to HESS J1800-240 given the large EGRET point spread function. The VHE differential photon spectra are well fit by pure power laws with indices ∼2.3 to 2.7. The spectral indices of HESS J1800-240A, B, and C are consistent within statistical errors. All VHE sources are ∼10 in intrinsic radius except for HESS J1800-240C, which appears pointlike. The NANTEN CO() data reveal molecular clouds positionally associating with the VHE emission, spanning a ∼15 km s range in local standard of rest velocity. Conclusions. The VHE/molecular cloud association could indicate a hadronic origin for HESS J1801-233 and HESS J1800-240, and several cloud components in projection may contribute to the VHE emission. The clouds have components covering a broad velocity range encompassing the distance estimates for W 28 (∼2 kpc) and extending up to ∼4 kpc. Assuming hadronic origin and distances of 2 and 4 kpc for cloud components, the required cosmic-ray density enhancement factors (with respect to the solar value) are in the range ∼10 to ∼30. If situated at 2 kpc distance, such cosmic-ray densities may be supplied by SNRs like W 28. Additionally and/or alternatively, particle acceleration may come from several catalogued SNRs and SNR candidates, the energetic ultra compact HII region W 28A2, and the HII regions M 8 and M 20, along with their associated open clusters. Further sub-mm observations would be recommended to probe in detail the dynamics of the molecular clouds at velocites $]]> 10 km s and their possible connection to W 28.

Original languageEnglish (US)
Pages (from-to)401-410
Number of pages10
JournalAstronomy and Astrophysics
Issue number2
StatePublished - Apr 2008


  • Gamma rays: observations

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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