A deep spectromorphological study of the $\gamma$-ray emission surrounding the young massive stellar cluster Westerlund 1
Creators: name: Aharonian, F.
name: Ashkar, H.
name: Backes, M.
name: Barbosa Martins, V.
name: Becherini, Y.
name: Berge, D.
name: Bi, B.
name: Böttcher, M.
name: de Bony de Lavergne, M.
name: Bradascio, F.
name: Brose, R.
name: Brun, F.
name: Bulik, T.
name: Burger-Scheidlin, C.
name: Cangemi, F.
name: Caroff, S.
name: Casanova, S.
name: Cerruti, M.
name: Chand, T.
name: Chandra, S.
name: Chen, A.
name: Chibueze, O.
name: Cristofari, P.
name: Mbarubucyeye, J. Damascene
name: Djannati-Ataï, A.
name: Ernenwein, J.-P.
name: Feijen, K.
name: de Clairfontaine, G. Fichet
name: Fontaine, G.
name: Funk, S.
name: Gabici, S.
name: Gallant, Y. A.
name: Ghafourizadeh, S.
name: Giavitto, G.
name: Giunti, L.
name: Glawion, D.
name: Glicenstein, J. F.
name: Goswami, P.
name: Grondin, M.-H.
name: Härer, L. K.
name: Haupt, M.
name: Hinton, J. A.
name: Hörbe, M.
name: Hofmann, W.
name: Holch, T. L.
name: Holler, M.
name: Horns, D.
name: Jamrozy, M.
name: Joshi, V.
name: Jung-Richardt, I.
name: Kasai, E.
name: Katarzynski, K.
name: Katz, U.
name: Khélifi, B.
name: Klúzniak, W.
name: Komin, Nu.
name: Kosack, K.
name: Kostunin, D.
name: Mezek, G. Kukec
name: Lang, R. G.
name: Le Stum, S.
name: Lemière, A.
name: Lemoine-Goumard, M.
name: Lenain, J.-P.
name: Leuschner, F.
name: Lohse, T.
name: Luashvili, A.
name: Lypova, I.
name: Mackey, J.
name: Majumdar, J.
name: Malyshev, D.
name: Marandon, V.
name: Marchegiani, P.
name: Marcowith, A.
name: Martí-Devesa, G.
name: Marx, R.
name: Maurin, G.
name: Meyer, M.
name: Mitchell, A.
name: Moderski, R.
name: Mohrmann, L.
name: Montanari, A.
name: Moulin, E.
name: Muller, J.
name: Murach, T.
name: Nakashima, K.
name: de Naurois, M.
name: Nayerhoda, A.
name: Niemiec, J.
name: Ohm, S.
name: Olivera-Nieto, L.
name: de Ona Wilhelmi, E.
name: Ostrowski, M.
name: Panny, S.
name: Panter, M.
name: Parsons, R. D.
name: Peron, G.
name: Prokhorov, D. A.
name: Pühlhofer, G.
name: Punch, M.
name: Quirrenbach, A.
name: Rauth, R.
name: Reichherzer, P.
name: Reimer, A.
name: Reimer, O.
name: Renaud, M.
name: Reville, B.
name: Rieger, F.
name: Rowell, G.
name: Rudak, B.
name: Ruiz-Velasco, E.
name: Sahakian, V.
name: Salzmann, H.
name: Sanchez, D. A.
name: Santangelo, A.
name: Sasaki, M.
name: Schüssler, F.
name: Schutte, H. M.
name: Schwanke, U.
name: Shapopi, J. N. S.
name: Specovius, A.
name: Spencer, S.
name: Stawarz, Ł.
name: Steenkamp, R.
name: Steinmassl, S.
name: Steppa, C.
name: Sushch, I.
name: Suzuki, H.
name: Takahashi, T.
name: Tanaka, T.
name: Terrier, R.
name: Thorpe-Morgan, C.
name: Tsirou, M.
name: Tsuji, N.
name: Tuffs, R.
name: Unbehaun, T.
name: van Eldik, C.
name: van Soelen, B.
name: Vecchi, M.
name: Veh, J.
name: Venter, C.
name: Vink, J.
name: Wagner, S. J.
name: White, R.
name: Wierzcholska, A.
name: Wong, Yu Wun
name: Zacharias, M.
name: Zargaryan, D.
name: Zdziarski, A. A.
name: Zhu, S. J.
name: Zouari, S.
name: Zywucka, N.
name: Blackwell, R.
name: Braiding, C.
name: Burton, M.
name: Cubuk, K.
name: Filipovic, M.
name: Tothill, N.
name: Wong, G.
Publisher: name: Deutsches Elektronen-Synchrotron, DESY, Hamburg
Year: 2022
Subjects: subject: 520
subject: acceleration of particles
subject: radiation mechanisms: non-thermal
subject: shock waves
subject: stars: massive
subject: gamma rays: general
subject: galaxies: star clusters: individual: Westerlund 1

description: 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 (∼2° 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.