Investigating particle acceleration in the Wolf-Rayet bubble NGC 2359

¹ Indian Institute of Space Science and Technology, Thiruvananthapuram 695 547, Kerala, India
² Space sciences, Technologies and Astrophysics Research (STAR) Institute, University of Liège, Liège, Belgium
³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
⁴ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden
⁵ Instituto Argentino de Radioastronomía (CONICET–CICPBA–UNLP), C.C No 5., 1894 Villa Elisa, Argentina
⁶ National Centre for Radio Astrophysics, Pune 411 007, Maharashtra, India

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 August 2025
Accepted: 1 February 2026

Abstract

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 ∼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.