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

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-energyγ-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 theγ-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 theγ-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)γ-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation withγ-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 theγ-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 theγ-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.