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A broadband view of the thermal and non-thermal emission from the embedded massive star cluster RCW 38

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Giada Peron, Andrea Bracco, Giovanni Morlino, Silvia Mantovanini, Elena Amato, Daniele Galli, Marco Padovani

High Energy Astrophysical Phenomena · Submitted 19 Jun 2026 · 290 references

Pith's verdict: RCW 38 paper compiles the widest radio-to-gamma data set for any star cluster and fits one 8-parameter MCMC model to report K_ep ≲ 10^{-3} and efficiency ≳1%, but skips comparisons to leptonic or multi-component alternatives.

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arXiv ID2606.21288 (version 1)
Pith IDZN7LGWUS, Pith's permanent record of this paper and its review
CitationAndrea Bracco, Daniele Galli, Elena Amato, Giada Peron, Giovanni Morlino, Marco Padovani, Silvia Mantovanini. "A broadband view of the thermal and non-thermal emission from the embedded massive star cluster RCW 38." arXiv:2606.21288 (2026). https://pith.science/paper/ZN7LGWUS
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The paper

The paper claims

Gamma-ray emission from RCW 38 arises from hadronic interactions with its molecular cloud, with electron-to-proton ratio below 0.001.

The reading

The paper presents a multi-wavelength analysis of RCW 38 from MHz radio through GeV gamma rays, fitting an eight-parameter model via MCMC to separate thermal and non-thermal components. It concludes that the gamma-ray flux is produced by protons accelerated in stellar winds colliding with the host molecular cloud rather than by electrons. This yields firm upper limits on the electron-to-proton ratio of 10 to the minus 3 and a lower limit on acceleration efficiency of 1 percent. The derived values match those needed to explain the galactic cosmic-ray composition including the neon-22 anomaly. The findings position young massive star clusters as significant sources of cosmic-ray protons at least up to a few TeV.

What carries the argument

An eight-parameter model of thermal and non-thermal emission constrained solely by MCMC fitting to observed fluxes from radio to gamma-ray bands.

Extended reading notes

Core claim

Our results support an interpretation in which the gamma-ray emission from RCW 38 is produced by hadronic interactions with the host molecular cloud. We derive robust constraints on the electron-to-proton ratio, with K_ep ≲ 10^{-3}, and on the acceleration efficiency, estimated to be ≳1%, consistent with the values required to explain the cosmic-ray composition, and in particular its 22Ne anomaly. These results strengthen the idea that stellar clusters play a significant role as contributors to cosmic-ray protons in our Galaxy at least up to energies of a few TeV.

Load-bearing premise

The entire broadband spectrum can be described by one eight-parameter model without significant extra source components or physical processes.

Editorial extensions

If this is right

  • Stellar-wind acceleration in clusters must operate with efficiency above 1 percent to match cosmic-ray composition data.
  • Young massive clusters supply a non-negligible fraction of galactic cosmic-ray protons up to a few TeV.
  • Next-generation ground-based gamma-ray telescopes can test whether the same mechanism extends to the cosmic-ray knee.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Applying the same multi-wavelength fitting approach to other embedded clusters could identify additional hadronic gamma-ray sources.
  • The very low electron-to-proton ratio implies that cluster wind shocks preferentially accelerate protons over electrons.
  • If clusters are confirmed as TeV proton sources, they may account for part of the diffuse galactic gamma-ray background.
Original abstract

Gamma-ray emission has now been detected from a variety of source classes in the Galaxy, including clusters of young massive stars. RCW 38, a very young embedded massive star cluster, is a case of particular interest: its gamma-ray emission detected up to hundreds of GeV, provided the first observational evidence of high-energy particle acceleration powered exclusively by stellar winds. In this work, we aim to characterize the emission mechanisms responsible for the gamma-ray flux in RCW 38 and to provide estimates of the acceleration efficiency, as well as the fraction of accelerated electrons compared to protons, $K_{ep}$. We present the most comprehensive multi-wavelength study of a single star cluster to date. Our analysis ranges from MHz radio observations obtained with the GLEAM-X survey from the Murchison Widefield Array (MWA) to GeV gamma-ray data from Fermi-LAT, and includes GHz and THz measurements from Parkes, Planck, and IRAS. We model the thermal and non-thermal emission of RCW 38 using an eight-parameter model constrained by the Markov chain Monte Carlo method. Our results support an interpretation in which the gamma-ray emission from RCW 38 is produced by hadronic interactions with the host molecular cloud. We derive robust constraints on the electron-to-proton ratio, with $K_{ep} \lesssim 10^{-3}$, and on the acceleration efficiency, estimated to be $\gtrsim$1%, consistent with the values required to explain the cosmic-ray composition, and in particular its $^{22}$Ne anomaly. These results strengthen the idea that stellar clusters play a significant role as contributors to cosmic-ray protons in our Galaxy at least up to energies of a few TeV. Future investigations with the next generation of ground-based detectors will determine whether they also play a relevant role at higher energies, particularly in the context of the cosmic-ray knee.

Figures
Figure 1. RGB map of RCW 38 as detected by MWA within the GLEAM￾X survey (Mantovanini et al. 2025), in three different bands: 88 MHz (red), 114 MHz (green) and 155 MHz (blue). The grey contours repre￾sent emission unveiled by IRAS at 60 µm, while the white contours is the gamma-ray signal derived by Fermi-LAT above 1 GeV. In the latter case, the contours indicate significance level corresponding to a test￾statistics of 50,100, and 500. 3. Emission modeling In this Section, we describe the initial assumptions about the ambient parameters and particle distribution adopted for mod￾eling the…
Figure 2. Energy-loss and acceleration timescales of electrons are com￾pared. The latter are calculated considering the diffusion coefficient in three different turbulence scenarios, Bohm, Kolmogorov, and Kraich￾nan, as indicated in the figure legend. The open circles indicate, for each case, the maximum energy, defined by the points at which the timescales intercept. The grey area indicates an epoch longer than the age of RCW 38, and therefore represents a forbidden area in this plot. 3.2.1. Dust thermal radiation The thermal emission of dust can be described by a modified black-body…
Figure 3. In the top panel: the best-fit model for the broadband SED of RCW 38. Red points refer to the observations of the different instruments listed in the central inset, while the black upper limit indicates the level of the emission measured by Chandra, as explained in the text. The grey curve represent the sum of thermal and non-thermal components, listed in the figure legend, the grey area indicates the confidence levels corresponding to the 16th and 84th percentiles. In the lower panel: the residuals of the fit computed as explained in the text. magnetic field is consistent with the…
Figure 4. Corner plots for the fitted parameters in the model. of Lw ∼ 1038 erg s−1 was assumed. Other authors have instead obtained for RCW 38 a much larger value for the acceleration efficiency (Pandey et al. 2024), because they assumed a much shorter confinement time, dominated by diffusion with a Galac￾tic diffusion coefficient. We assumed throughout the text that ad￾vection dominates up to energies ∼ 1 TeV, but we stress that if diffusion time were shorter than advection,the efficiency would increase accordingly. We can hence look at our value as a con￾servative lower limit to the…
Figure 5. Electrons (orange) and protons (blue) spectral energy distribu￾tion (as a function of total energy of the particles) calculated including the terms of losses with the best-fit parameters listed in
Figure 6. CTAO sensitivity of the Southern array compared with the SED of RCW 38 assuming different maximum energies of primary particles. The sensitivity is calculated for an exposure of 200 hours on-axis, using the prescription of Celli & Peron (2024), namely requiring a minimum 10 events per bin with a significance of 5σ over the background. The different golden curves represent the sensitivity for a point like source (dotted line) and for an extended source with radius ranging from 0.1◦ to 0.2◦ (area). massive stars are able to contribute a small fraction of cosmic rays at Earth. A…

Figures reproduced from arXiv:2606.21288 by the authors.

References

How the literature uses it

What this paper does with the arXiv papers it cites

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

What it cites · 12 sentences without a confident label

Uses the method or tool of 2 papers
Uses data or values from 8 papers
  • At meter wavelengths, we used the recently released Galactic Plane (GP) data [ref] obtained with the Murchison Widefield Array [ref] as part of the Galactic and Extragalactic All-Sky MWA eXtended survey [ref] , which covers the southern GP ($233 < l < 44$ and $|b| < 11$) over a frequency range of $72 - 231$ MHz divided into 5 wide sub-bands ($≈30$ MHz) and 20 further narrow sub-bands ($≈8$ MHz).

    2602.08475 · Radio data · 0.94

  • At meter wavelengths, we used the recently released Galactic Plane (GP) data [ref] obtained with the Murchison Widefield Array [ref] as part of the Galactic and Extragalactic All-Sky MWA eXtended survey [ref] , which covers the southern GP ($233 < l < 44$ and $|b| < 11$) over a frequency range of $72 - 231$ MHz divided into 5 wide sub-bands ($≈30$ MHz) and 20 further narrow sub-bands ($≈8$ MHz).

    GaLactic and Extragalactic All-sky Murchison Widefield Array eXtended (GLEAM-X) survey II: Second Data Release · Radio data · 0.94

  • The latter was derived from the bolometric luminosity of the 29 OB star candidates identified by [ref] , with $U_rad,ts=∑_i L_bol,i /(4 πc R^2_ts)∼290$ eV cm$^-3$, assuming the same peak luminosity of 30,000 K for all stars.

    X-ray and IR Point Source Identification and Characteristics In the Embedded, Massive Star-Forming Region RCW 38 · Inverse Compton · 1.00

Compares with 1 paper
Cites as background 3 papers
  • Particles in these environments could be energized in different ways: via diffusive shock acceleration (DSA) at the wind-wind collision shock of binary stellar systems [ref] or at the termination shock of the collective wind [ref] ; or via the second-order Fermi mechanism, induced by the scattering of the particles on the turbulent magnetic fields found in the wind blown bubble [ref] .

    Superbubbles as Galactic PeVatrons: The Potential Role of Rapid Second-Order Fermi Acceleration · Introduction · 0.96

  • Particles in these environments could be energized in different ways: via diffusive shock acceleration (DSA) at the wind-wind collision shock of binary stellar systems [ref] or at the termination shock of the collective wind [ref] ; or via the second-order Fermi mechanism, induced by the scattering of the particles on the turbulent magnetic fields found in the wind blown bubble [ref] .

    Non-thermal high-energy emission from colliding winds of massive stars · Introduction · 0.96

  • The value of $η_B$ cannot be constrained with available information, although it is commonly assumed to be a small fraction [ref] of the wind power.

    Inside the core of a young massive star cluster: 3D MHD simulations · Primary particles · 0.85

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