Detection of diffuse gamma-ray emission towards a massive star forming region hosting Wolf-Rayet stars

Abstract

Isotopic and elemental abundances seen in Galactic cosmic rays imply that $\sim 20\%$ of the cosmic-ray (CR) nuclei are probably synthesized by massive Wolf-Rayet (WR) stars. Massive star clusters hosting WR and OB-type stars have been proposed as potential Galactic cosmic-ray accelerators for decades, in particular via diffusive shock acceleration at wind termination shocks. Here we report the analysis of {\em Fermi} Large Area Telescope's data towards the direction of Masgomas-6a, a young massive star cluster candidate hosting two WR stars. We detect an extended $\gamma$-ray source with $\mathrm{TS}=183$ in the vicinity of Masgomas-6a, spatially coincident with two unassociated {\em Fermi} 4FGL sources. We also present the CO observational results of molecular clouds in this region, using the data from the Milky Way Imaging Scroll Painting project. The $\gamma$-ray emission intensity correlates well with the distribution of molecular gas at the distance of Masgomas-6a, indicating that these gamma rays may be produced by CRs accelerated by massive stars in Masgomas-6a. At the distance of $3.9\mathrm{kpc}$ of Masgomas-6a, the luminosity of the extended source is $(1.81\pm 0.02)\times 10^{35}\mathrm{erg}{\mathrm{s}}^{-1}$. With a kinetic luminosity of $\sim 10^{37}\mathrm{erg}{\mathrm{s}}^{-1}$ in the stellar winds, the WR stars are capable of powering the $\gamma$-ray emission via neutral pion decay resulted from cosmic ray $pp$ interactions. The size of the GeV source and the energetic requirement suggests a CR diffusion coefficient smaller than that in the Galactic interstellar medium, indicating strong suppression of CR diffusion in the molecular cloud.

11 pages, 5 figures and 1 table. Accepted for publication in ApJ

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Cited by in corpus (1)

• Exploring the nature of Galactic unassociated sources detected by the Fermi-LAT