Investigating particle acceleration in the Wolf-Rayet bubble NGC 2359

Journal article, 2026

Context. Massive stars have been proposed as potential major factories of Galactic cosmic rays (GCRs). However, this claim lacks sufficient empirical evidence, especially in the case of isolated stars. The powerful stellar winds from massive stars impact the ambient medium and produce strong shocks suitable for accelerating relativistic particles. The detection of nonthermal emission - particularly synchrotron emission in low-frequency radio bands - serves as proof of particle acceleration sites. Aims. We assess whether isolated massive stars can be sources of GCRs. Methods. We observed the Wolf-Rayet bubble NGC 2359 using the upgraded Giant Metrewave Radio Telescope in Band 3 (250-500 MHz) and Band 4 (550-950 MHz). Additionally, we utilized complementary archival radio datasets across different frequencies to derive the broad spectral energy distribution (SED) for several regions within the bubble. In addition, to further characterize the interaction between the stellar wind and the ambient medium, we introduced a composite SED model that includes synchrotron and free-free emission, as well as two low-frequency turnover processes, the Razin-Tsytovich (RT) effect and free-free absorption (FFA). We used a Bayesian inference approach to fit the SEDs and constrain the electron number density and magnetic field strength. Results. The SEDs of several regions across the bubble have spectral indices steeper than -0.5, indicative of synchrotron radiation. Furthermore, the SEDs show a turnover below similar to 1 GHz. Our SED modeling suggests that the observed turnover is primarily caused by the RT effect, with a minor contribution from internal FFA. Conclusions. Our analysis confirms the presence of synchrotron radiation within NGC 2359. This is the second detection of nonthermal emission in a stellar bubble surrounding a Wolf-Rayet star, reinforcing the idea that such environments are sites of relativistic particle acceleration. This finding further supports the hypothesis that isolated massive stars are sources of GCRs of at least GeV energies.