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

¹ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
² Max-Planck-Institut für Kernphysik, PO Box 103980 69029 Heidelberg, Germany
³ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, 91128 Palaiseau, France
⁴ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, 10005, Namibia
⁵ Centre for Space Research, North-West University, Potchefstroom, 2520, South Africa
⁶ DESY, 15738 Zeuthen, Germany
⁷ Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁸ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
⁹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹⁰ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
¹¹ Université Savoie Mont Blanc, CNRS, Laboratoire d’Annecy de Physique des Particules – IN2P3, 74000 Annecy, France
¹² IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹³ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
¹⁴ Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Énergies, LPNHE, 4 Place Jussieu, 75252 Paris, France
¹⁵ Instytut Fizyki Jdrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
¹⁶ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050, South Africa
¹⁷ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
¹⁸ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
¹⁹ School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia
²⁰ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
²¹ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France
²² Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²³ Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, 33170 Gradignan, France
²⁴ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
²⁵ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
²⁶ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
²⁷ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
²⁸ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
²⁹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
³⁰ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³¹ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
³² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
³³ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada, Kobe, Hyogo 658-8501, Japan
³⁴ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8583, Japan
³⁵ RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³⁶ Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
³⁷ School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
³⁸ Armagh Observatory and Planetarium, Armagh BT61 7HT, UK
³⁹ Western Sydney University, Locked Bag 1797, Penrith South DC, NSW 2751, Australia

Received: 22 June 2022
Accepted: 21 July 2022

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 very-high-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 h 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.