From direct observations, we know that our Galaxy, the Milky Way, contains sources of cosmic rays with energies of up to several hundred Peta-electronvolts (1015 eV). The acceleration of atomic nuclei to such extreme energies requires very special astrophysical environments. Young massive star clusters – compact aggregations of stars with tens to hundreds of solar masses – have been proposed as excellent candidates for such environments.
Indeed, the most massive young star cluster in the Milky Way, Westerlund 1, was recently found to be a powerful cosmic-ray accelerator. Now, scientists at the Max Planck Institute for Nuclear Physics have identified another star cluster – named R136 – as a source of cosmic rays, using the H.E.S.S. gamma-ray observatory. Lars Mohrmann, leader of the H.E.S.S. group at MPIK and lead author of the study, explains: “What is remarkable about this detection is that R136 is located in the Large Magellanic Cloud, and therefore about ten times further away from us than Westerlund 1.” The new gamma-ray source associated with R136 is named “HESS J0538–691”.
A big challenge in the analysis of the data was that HESS J0538–691 lies very close to the already known gamma-ray source HESS J0537–691. This source, related to the pulsar wind nebula N 157B, is much brighter than HESS J0538–691 and partially overlaps with the new source (see the second panel in Figure 1). Only a careful modelling of the gamma-ray emission made it possible to subtract the signal from HESS J0537–691 and thus reveal the newly discovered source (bottom panel in Figure 1). Shown in the figure as well is another gamma-ray source named HESS J0535–691. This source, also associated with a massive star cluster named LH 90, had already been detected previously but could be studied in more detail thanks to the new analysis.
Both sources are extraordinarily bright in the gamma ray spectrum – they shine more than twice as bright as Westerlund 1, the most massive young star cluster in our Galaxy. Another interesting outcome of the study is that the emission from both sources appears spatially extended, with a radius of about 100 light years. This could indicate that the acceleration of cosmic rays is connected to the collective wind that forms around the cluster as a superposition of the individual stellar winds. The collective wind creates shock waves at which particles can be accelerated. In addition, the wind generates a "superbubble", a cavity in the interstellar medium, whose expected size is approximately in agreement with the extension of the gamma-ray emission. In the paper the authors discuss possible processes for the acceleration of the cosmic rays, and thus the origin of the gamma-ray emission.
The detection of gamma-ray emission from two young massive star clusters in the Large Magellanic Cloud lends further support to the hypothesis that these objects are major contributors to the flux of high-energy cosmic rays, and motivates searching for emission from other star clusters in the Milky Way.
Original publication:
Very-high-energy γ-ray emission from young massive star clusters in the Large Magellanic Cloud, F. Aharonian et al. (H.E.S.S. Collaboration), Astrophysical Journal Letters 970, L21, accepted (2024). DOI: doi.org/10.3847/2041-8213/ad5e67
Weblinks:
Website of the H.E.S.S.-Experiment
https://www.aanda.org/articles/aa/abs/2022/10/aa44323-22/aa44323-22.html