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Title: Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field
Authors: Aharonian, F.
Benbow, W.
Berge, D.
Bernlöhr, K.
Bolz, O.
Braun, I.
Bühler, R.
Carrigan, S.
Clapson, A. C.
Costamante, L.
Domainko, W.
Rowell, G.
Van Eldik C
Völk, H. J.
Akhperjanian, A. G.
Kerschhaggl, M.
Sahakian, V.
Bazer-Bachi, A. R.
Borrel, V.
Vivier, M.
Olive, J. -. P.
Dubus, G.
Beilicke, M.
Cornils, R.
Heinzelmann, G.
Raue, M.
Ripken, J.
Brown, A. M.
Lohse, T.
Schlenker, S.
Schwanke, U.
Boisson, C.
Fontaine, G.
Lenain, J. -. P.
Espigat, P.
Martin, J. M.
Sol, H.
Brion, E.
Glicenstein, J. F.
Goret, P.
Chadwick, P. M.
Dickinson, H. J.
Hadjichristidis, C.
Giebels, B.
Latham, I. J.
Le Gallou R
Lemière, A.
McComb, T. J. L.
Pühlhofer, G.
Nolan, S. J.
Orford, K. J.
Osborne, J. L.
Rayner, S. M.
Spangler, D.
Khélifi, B.
Ward, M.
Büsching, I.
Holleran, M.
Maurin, G.
De Jager OC
Raubenheimer, B. C.
Schwemmer, S.
Venter, C.
Chounet, L. -. M.
Degrange, B.
Pita, S.
Lemoine-Goumard, M.
Superina, G.
Volpe, F.
Tam, P. H.
Coignet, G.
Santangelo, A.
Lamanna, G.
Ranchon, S.
Rosier-Lees, S.
Vialle, J. P.
Djannati-Ataï, A.
Wagner, S. J.
Punch, M.
Terrier, R.
Aharonian, F.
Drury, L. O.
Schwarzburg, S.
Masterson, C.
Ostrowski, M.
Behera, B.
Emmanoulopoulos, D.
Hauser, M.
Hinton, J. A.
Pedaletti, G.
Farnier, C.
Feinstein, F.
Fiasson, A.
Martineau-Huynh, O.
Gallant, Y. A.
Jacholkowska, A.
Stawarz, Ł.
Komin, N.
Egberts, K.
Marcowith, A.
Moulin, E.
Vasileiadis, G.
Glück, B.
Stegmann, C.
De Naurois M
Boutelier, T.
Henri, G.
Pelletier, G.
Bulik, T.
Petrucci, P. -. O.
Saugé, L.
Förster, A.
Hoffmann, A.
Horns, D.
Kendziorra, E.
Dyks, J.
Tavernet, J. -. P.
Vincent, P.
Nedbal, D.
Funk, S.
Rob, L.
Bernlöhr, K.
Ruppel, J.
Schlickeiser, R.
Schroder, R.
Shalchi, A.
Steenkamp, R.
Hauser, D.
Moderski, R.
Rudak, B.
Costamante, L.
Fukui, Y.
Funk, S.
Moriguchi, Y.
Moulin, E.
Takeuchi, T.
Berge, D.
Hinton, J. A.
Reimer, O.
Rowell, G.
Hermann, G.
Hinton, J. A.
Hofmann, W.
Füßling, M.
Hoppe, S.
Khélifi, B.
Rolland, L.
Kosack, K.
Panter, M.
Renaud, M.
First Published: Apr-2008
Publisher: EDP Sciences for European Southern Observatory (ESO)
Citation: Astronomy & Astrophysics, 2008, 481 (2), pp. 401-410
Abstract: Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV $\gamma$-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) $\gamma$-ray energies (E > 0.1 TeV) with the HESS. Cherenkov telescopes. A reanalysis of EGRET E > 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 $\gamma$-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 $\Gamma$ ~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$^\prime$ in intrinsic radius except for HESS J1800-240C, which appears pointlike. The NANTEN 12CO(J = 1-0) data reveal molecular clouds positionally associating with the VHE emission, spanning a ~15 km s-1 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-1 and their possible connection to W 28.
DOI Link: 10.1051/0004-6361:20077765
ISSN: 0004-6361
eISSN: 1432-0746
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2008 ESO. Reproduced with permission from Astronomy & Astrophysics, © ESO.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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