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Type: Journal article
Title: Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W 28 (G6.4-0.1) field
Author: Aharonian, F.
Akhperjanian, A.
Bazer-Bachi, A.
Behera, B.
Beilicke, M.
Benbow, W.
Berge, D.
Bernlohr, K.
Boisson, C.
Bolz, O.
Borrel, V.
Braun, I.
Brion, E.
Brown, A.
Buhler, R.
Bulik, T.
Busching, I.
Boutelier, T.
Carrigan, S.
Chadwick, P.
et al.
Citation: Astronomy and Astrophysics, 2008; 481(2):401-410
Publisher: EDP Sciences
Issue Date: 2008
ISSN: 0004-6361
Statement of
F. Aharonian ... G. Rowell ... et al.
Abstract: Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV γ-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) γ-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 γ-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 Γ ~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 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.
Keywords: gamma rays: observations
Rights: © ESO 2008. Article published by EDP Sciences
RMID: 0020083659
DOI: 10.1051/0004-6361:20077765
Appears in Collections:Chemistry and Physics publications

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