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Aharonian, F.; Ashkar, H.; Becherini, Yvonne; Berge, D.; Bottcher, M.; de Lavergne, M. de Bony; Bulik, T.; Cangemi, F.; Caroff, S.; Casanova, S.; Cerruti, M.; Chand, T.; Chandra, S.; Chen, A.; Chibueze, O.; Cristofari, P.; Mbarubucyeye, J. Damascene; Djannati-Atai, A.; de Clairfontaine, G. Fichet; Fontaine, G.; Funk, S.; Gabici, S.; Giavitto, G.; Giunti, L.; Glawion, D.; Goswami, P.; Grondin, M-H; Haerer, L. K.; Hinton, J. A.; Hofmann, W.; Holch, T. L.; Holler, M.; Horns, D.; Jamrozy, M.; Jung-Richardt, I; Katarzynski, K.; Katz, U.; Komin, Nu; Kosack, K.; Kostunin, D.; Kukec Mezek, Gasper; Lang, R. G.; Le Stum, S.; Luashvili, A.; Lypova, I; Majumdar, J.; Malyshev, D.; Marandon, V; Marti-Devesa, G.; Marx, R.; Mitchell, A.; Moderski, R.; Mohrmann, L.; Montanari, A.; Moulin, E.; Nayerhoda, A.; Olivera-Nieto, L.; Wilhelmi, E. de Ona; Ostrowski, M.; Panny, S.; Panter, M.; Parsons, R. D.; Peron, G.; Prokhorov, D. A.; Punch, Michael; Quirrenbach, A.; Rauth, R.; Reichherzer, P.; Reimer, A.; Reimer, O.; Renaud, M.; Rudak, B.; Sanchez, D. A.; Schwanke, U.; Steenkamp, R.; Sushch, I; Takahashi, T.; Tanaka, T.; Terrier, R.; Thorpe-Morgan, C.; Tsirou, M.; Tsuji, N.; Unbehaun, T.; van Eldik, C.; Vecchi, M.; Veh, J.; Vink, J.; Wagner, S. J.; Wierzcholska, A.; Wong, Y. Wun; Zacharias, M.; Zdziarski, A. A.; Zhu, S. J.; Zouari, S.; Zywucka, N.; Braiding, C.; Burton, M.; Filipovic, M.; Tothill, N.; Wong, G.
Astronomy and astrophysics (Berlin), 2022, Volume: 666Journal Article
Context. Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy gamma-ray source HESS J1646-458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified. Aims. We aim to identify the physical processes responsible for the gamma-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better. Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the gamma-ray emission of HESS J1646-458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs. Results. We detected large-scale (similar to 2 degrees diameter) gamma-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with gamma-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the gamma-ray emission with gas clouds as identified through H I and CO observations. Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the gamma-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.
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