A deep spectromorphological study of the γ-ray emission surrounding the young massive stellar cluster Westerlund 1

Abstract

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 veryhigh-energy γ-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 γ-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 hours of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the γ-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 with γ-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 γ-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 γ-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.

Inside the core of a young massive star cluster: 3D MHD simulations

“…Some clusters were also detected in very high energy gamma-rays (e.g. Aharonian et al 2007Aharonian et al , 2022Ohm et al 2010;Yang et al 2018) which is a clear indication of efficient cosmic ray (CR) acceleration processes in these sources which can be studied in detail with the forthcoming Cherenkov Telescope Array observatory (see e.g. Acero et al 2023;Acharyya et al 2023).…”

Particle acceleration in superbubbles: MHD simulations and $\gamma$-ray signatures

“…A recent analysis of H.E.S.S. data revealed ring-like TeV -ray emission around Westerlund 1 [2] (designated HESS J1646−458, see Fig. 1), following the detection of the source in 2012 [5].…”

Young massive stellar clusters as cosmic-ray sources: the case of Westerlund 1

“…We have presented the results of a recent study of the 𝛾-ray emission of HESS J1646−458, which surrounds the young massive stellar cluster Westerlund 1 [9]. HESS J1646−458 is a largely extended (∼2 • ) 𝛾-ray source with a very complex morphology.…”

“…The total required energy in protons above an energy of 1 GeV is 𝑊 𝑝 = 6 × 10 51 (𝑛/1 cm −3 ) −1 erg, where 𝑛 is the density of the ambient medium. For the leptonic model, we obtained a primary electron spectrum with index Γ 𝑒 = 2.97 ± 0.07 and cut-off energy 𝐸 𝑒 𝑐 = (180 +200 −70 ) TeV, where we have used a target photon field related to the stellar cluster in addition to the usually adopted infrared and cosmic microwave background photon fields (see [9] for details). The required luminosity in electrons above 0.1 TeV is at least 𝐿 𝑒 > 4.1 × 10 35 erg s −1 , and increases in the presence of magnetic fields to due synchrotron losses of the electrons.…”

“…Here, we summarise results from a recently published, updated study of the 𝛾-ray emission surrounding Westerlund 1 with the H.E.S.S. telescopes [9]. All results and figures presented in this proceedings article are based on this study, to which the reader is referred for further details.…”