Determine whether Westerlund 1 can produce the Source C and Source D emission

Determine whether the soft extended gamma-ray emission from Source C and Source D is produced by particles accelerated or reaccelerated in the Westerlund 1 superbubble, rather than by large-scale cosmic rays interacting with poorly modeled interstellar gas.

Background

Source C and Source D lie near gas structures at the edge of the Westerlund 1 superbubble and can be modeled either as extended disks or as gas-correlated templates. Their flux is energetically compatible with a small fraction of the cluster wind power, but the same emission could arise from shortcomings in the interstellar background model.

The morphology does not correspond perfectly to the gas templates, and the required particle spectrum differs from that inferred for the TeV emission. Consequently, the paper does not establish whether Westerlund 1 is the source of these components or whether the emission instead reflects background-gas modeling errors.

References

This is seen as strong evidence supporting the conjecture that CRs are efficiently accelerated in massive star-forming regions where a significant amount of energy from strong stellar winds and supernova explosions is passed to CRs.

— Gamma-ray emission toward supergiant shells in the Large Magellanic Cloud  (2609.28164 - Yu et al., 23 Sep 2026) in Section 1, Introduction

However, our results do not make it possible to reach firm conclusions. To this end, more detailed physical modeling and more accurate observational constraints are needed.

— Extended gamma-ray emission in the vicinity of the Westerlund 1 massive star cluster and Kes 41 supernova remnant seen by the Fermi Large Area Telescope  (2609.10176 - Tibaldo et al., 9 Sep 2026) in Section 5.2, Source C and Source D; Appendix B, Section 7

We note that an inverse-Compton origin of the emission cannot be excluded, but would also require a drop in the particle spectrum at energies <100 GeV.

— Extended gamma-ray emission in the vicinity of the Westerlund 1 massive star cluster and Kes 41 supernova remnant seen by the Fermi Large Area Telescope  (2609.10176 - Tibaldo et al., 9 Sep 2026) in Section 5.2, Source C and Source D