Investigating particle acceleration in the Wolf-Rayet bubble NGC 2359

TL;DR

The paper tests the hypothesis that isolated massive stars can accelerate particles to Galactic cosmic ray energies by inspecting the WR bubble NGC 2359 around WR 7. It combines new low-frequency uGMRT data with archival radio observations and applies a Bayesian, composite SED model that includes synchrotron and free-free emission plus two turnover mechanisms (Razin–Tsytovich effect and internal free–free absorption) to constrain the electron density $n_{ m e}$ and magnetic field strength $B$. The analysis reveals clear synchrotron emission and a low-frequency turnover largely due to RT, with $B$ in the $\sim$0.5–4 $\mu$G range and $n_{ m e}$ substantially lower than some previous estimates; these findings substantiate particle acceleration in wind–ISM interactions around single WR stars, though the energy balance raises questions about the required non-thermal electron population. The work highlights the value of broad, low-frequency radio coverage and points to the need for larger samples of WR bubbles to better understand the conditions that drive shock-accelerated particles, especially in the SKA era.

Abstract

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.

Figures (7)

• Figure 1: Maps of NGC 2359 in optical and IR. (a) Optical Morphology of NGC 2359 from the Digitized Sky Survey 2 (DSS2). (b) Spitzer colour-composite (3.6 $\mu$m (blue), 4.5 $\mu$m (green), and 8.0 $\mu$m (red) bands) image. The arrow indicates the direction of motion of WR 7 with respect to the local medium (see Section \ref{['sec:results-morphology']} for details).
• Figure 2: Radio maps of NGC 2359 obtained using our uGMRT data. (a) Maps obtained using the full-band data. (b) Sub-band images for Band-4 data. All maps are convolved to a circular beam of 22. For each map, the central frequency and rms noise ($\sigma$) in units of $\rm mJy\,beam^{-1}$ are mentioned in each panel. The 'X' marks the location of the star in all the panels. Identified apertures (E1--E4) and the central region for obtaining the SEDs are shown in the 735 MHz map. Contour levels are as follows: (i) For maps with central frequencies of 402 and 735 MHz, contour levels start from 3$\sigma$ and increase in steps of 4$\sigma$. (ii) The maps with central frequencies of 622, 657, 692, and 728 MHz, have contour levels of [3, 8, 13, 28, 43, 58, 73, 88, 103, 118] $\times\,\sigma$. (iii) For maps with central frequencies of 761 and 801 MHz, the contour levels are [3, 8, 13, 28, 43, 58, 73] $\times\,\sigma$. In all panels, the contours are smoothed over 3 pixels using a Gaussian kernel.
• Figure 3: Radio maps of NGC 2359 obtained using archival data. For each map, the central frequency and rms noise ($\sigma$) in units of $\rm mJy\,beam^{-1}$ are mentioned in each panel. Barring the image at 150 MHz, all maps are convolved to a circular beam of 22. The 150 MHz map has a beam size of 29.1$\times$ 24.9 and is smoothed across 3 pixels using a Gaussian kernel. The 'X' marks the location of the star in all the panels. Identified apertures (E1--E4) and the central region for obtaining the SEDs are shown in the 1425, 4860, and 8689 MHz maps. Contour levels are as follows: (i) For maps with the central frequency of 150 MHz, contour levels start from 2$\sigma$ and increase in steps of 1$\sigma$. (ii) For maps with central frequencies of 887 and 943 MHz, contour levels start from 3$\sigma$ and increase in steps of 5$\sigma$, (iii) The 1425 MHz map has contour levels of [3, 7, 11, 15, 23, 31, 39, 47, 54, 62, 70, 78] $\times\,\sigma$. (iv) For maps with central frequencies of 4860 and 8689 MHz, contour levels start from 3$\sigma$ and increase in steps of 4$\sigma$. In all panels, the contours are smoothed over 3 pixels using a Gaussian kernel.
• Figure 4: Radio SED for the identified apertures (E1--E4) shown in Figures \ref{['fig:WR7-ALL-uGMRT-radiomaps']} and \ref{['fig:WR7-ALL-archive-radiomaps']}. (a) SED using the maps at 150, 402, 735, 887, 943, 1425, 4860, and 8689 MHz maps. The data point at 402 MHz (Band 3) is a lower limit (see Section \ref{['sec:WR7-uGMRT-dataanalysis']} for details). (b) SED using the maps of the six sub-bands (622, 657, 692, 728, 761, and 801 MHz) of the Band 4 GWB uGMRT data. The uncertainty in the flux density measurements is estimated using Lal2021$\Delta S = [ (S_\nu \times f )^{2} + rms^{2} \times N_{\rm beams} ]^{0.5}$, where $S_\nu$ is the flux density, $f$ is an absolute flux density calibration uncertainty (taken as 10%), and $N_{\rm beams}$ is the number of synthesized beams in the aperture.
• Figure 5: SED fitting for apertures E1--E4 and the central region using the composite model described in Equation \ref{['eq:model-composite']}. The shaded regions show the 1$\sigma$ confidence interval. The bottom panels of each image present the residuals of the fit. The corresponding corner plots showing the posterior distribution of each parameter are presented in the Appendix \ref{['sec:appendix-A']}.