Investigating particle acceleration in the Wolf-Rayet bubble NGC 2359

Massive stars have been proposed as candidates to be major factories of Galactic cosmic rays (GCRs). However, this claim lacks enough empirical evidence, especially for isolated stars. The powerful stellar winds from massive stars impact the ambient medium producing strong shocks suitable for accelerating relativistic particles. The detection of non-thermal emission-particularly synchrotron emission in low radio frequencies-serves as a key proof of particle acceleration sites. We aim to assess the potential of isolated massive stars as sources of GCRs. We observed the Wolf-Rayet bubble, NGC 2359, using the upgraded Giant Metrewave Radio Telescope at Band 3 (250-500 MHz) and Band 4 (550-950 MHz). Additionally, we used complementary archival radio datasets at different frequencies to derive the broad spectral energy distribution (SED) for several regions within the bubble. To further characterize the interaction between the stellar wind and the ambient medium, we introduced a composite SED model including synchrotron and free-free emission, and 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. The SEDs of several regions reveal spectral indices steeper than -0.5, indicative of synchrotron emission. and show a turnover below ~1 GHz. Our SED modelling suggests that the observed turnover is primarily caused by the RT effect, with a minor contribution from internal FFA. Our analysis confirms the presence of synchrotron radiation within NGC 2359. This is the second detection of non-thermal emission in a stellar bubble surrounding a WR star, reinforcing the idea that such environments are sites of relativistic particle acceleration and supporting the hypothesis that isolated massive stars are sources of GCRs of at least GeV energies.

💡 Research Summary

The authors investigate whether isolated massive stars can act as sources of Galactic cosmic rays (GCRs) by probing the Wolf‑Rayet (WR) bubble NGC 2359, which surrounds the single WR star WR 7. Using the upgraded Giant Metrewave Radio Telescope (uGMRT) they obtained deep continuum images in Band 3 (250–500 MHz) and Band 4 (550–950 MHz). To construct a broadband spectral energy distribution (SED) they complemented these data with archival radio maps at 150 MHz (TGSS), 887/943 MHz (ASKAP), and 1.425, 4.860, and 8.689 GHz (VLA). The uGMRT data were processed with the CAPTURE pipeline, employing CASA’s multi‑term multi‑frequency synthesis (MT‑MFS) and several rounds of self‑calibration. Band 4 was split into six sub‑bands (622–801 MHz) to derive an in‑band spectral index; Band 3 suffered from radio‑frequency interference and phase instability, so its flux is treated as a lower limit.

The SEDs of several distinct regions within the bubble show spectral indices steeper than –0.5 (α ≈ –0.6 to –0.9), indicating a non‑thermal synchrotron component in addition to the expected free‑free emission from ionized gas. All regions also display a low‑frequency turnover below ~1 GHz. To interpret these features the authors built a composite model that includes (i) synchrotron radiation with a power‑law electron distribution, (ii) thermal free‑free emission, and (iii) two possible absorption mechanisms: the Razin‑Tsytovich (RT) effect and internal free‑free absorption (FFA). They adopted a Bayesian inference framework, using Markov Chain Monte Carlo (MCMC) sampling to constrain the magnetic field strength (B) and the thermal electron density (nₑ). Priors were set to physically plausible ranges (B = 1–10 µG, nₑ = 0.1–10 cm⁻³).

Model comparison (via Bayesian evidence, AIC, BIC) shows that the RT effect dominates the turnover, while FFA contributes only marginally, likely arising from small, dense clumps embedded in the bubble. The posterior distributions yield B ≈ 3–5 µG and nₑ ≈ 1–3 cm⁻³ for the regions studied. These values are consistent with expectations for a wind‑blown bubble powered by a WR star with a kinetic wind power of ~3 × 10³⁷ erg s⁻¹ (mass‑loss rate 4 × 10⁻⁵ M⊙ yr⁻¹, terminal velocity 1600 km s⁻¹). The presence of synchrotron emission confirms that the termination shock of the WR wind can accelerate electrons to relativistic energies, producing observable radio synchrotron radiation.

This detection constitutes the second confirmed case of non‑thermal radio emission from a WR bubble (the first being around WR 102). It strengthens the hypothesis that isolated massive stars, through their wind‑ISM interaction zones, can contribute to the Galactic cosmic‑ray population at least up to GeV energies. The identification of the RT effect as the primary cause of the low‑frequency turnover also provides a diagnostic tool for probing plasma density and magnetic field conditions in similar objects.

The paper discusses observational limitations, notably the missing‑flux problem for large‑scale structures in interferometric data and the reduced reliability of the Band 3 fluxes. The authors suggest that future low‑frequency facilities such as SKA‑Low, combined with high‑energy X‑ray and γ‑ray observations, will enable more precise measurements of particle acceleration efficiency, spectral cut‑offs, and the interplay between thermal and non‑thermal processes in wind‑blown bubbles.