Massive Star Clusters as sources of high-energy gamma radiation

TL;DR

This study links massive star clusters (MSCs) to high-energy gamma-ray production and potential ultra-high-energy cosmic-ray contributions in the Galaxy by modeling proton acceleration in two MSC classes: Soft clusters with isolated SN shocks and Powerful clusters with wind-driven, wind-terminal shocks. A hybrid population approach combines MSCs within $3\,\mathrm{kpc}$ from the Sun with a synthetic disk population to capture Galactic-scale contributions, predicting a three-component cosmic-ray spectrum that includes wind-driven PeV acceleration ($E_{ m wind}^{\max} \sim 1\,Z\,\mathrm{PeV}$) and wind+SN–driven PeV energies ($E_{ m pow}^{\max} \sim 4\,Z\,v_5$ PeV). Gamma-ray emission is computed via hadronic $p$-$p$ interactions using the Kelner et al. parameterization, with $\eta_{\rm H} \sim 10\ \mathrm{cm^{-3}}$ and a nuclear enhancement $\epsilon(E_p) \approx 1.15$, showing that nearby MSCs dominate the observed flux up to the TeV–PeV range and that wind shocks contribute negligibly to gamma rays compared to SNR shocks. The results align with high-energy observations, support a PeV-capable role for young compact clusters, and provide a framework for interpreting the gamma-ray sky in terms of resolved and unresolved MSC populations, with implications for multi-messenger astrophysics.

Abstract

This paper investigates the contribution of massive star clusters (MSC) as sources of high-energy gamma rays and their impact on the ultra-high-energy (UHE) emission observed throughout the Galaxy. By modeling proton injection, the study explores how the acceleration of protons in massive star clusters contributes to the gamma radiation detectable from Earth. The analysis focuses on two primary types of clusters: widespread, dispersed clusters and younger, compact massive clusters, both of which host shock waves generated by supernova remnants (SNR). Clusters located near the solar system, within a 3-kiloparsec radius,are identified. Analytical methods are used to calculate energy spectra and gamma-ray production rates. The findings suggest that young and compact MSC contribute to multi-TeV to PeV gamma-ray emission, with the dominant contribution arising from nearby populations.

Figures (8)

• Figure 1: Mollweide projection of stars from the 2013AA...558A..53K catalog, plotted in Galactic coordinates. Star clusters are shown in gray, while massive star clusters are highlighted in red.
• Figure 2: Distribution of star clusters from the MSC survey 2013AA...558A..53K is shown projected onto the Galactic XY plane, with distances measured relative to the Sun. The right panel displays a zoomed-in view of nearby clusters, where circle sizes scale with stellar membership. A green circle highlights sources within $3\, \mathrm{kpc}$ of the Solar System.
• Figure 3: Spatial distribution of MSC in the Milky Way, obtained by combining a synthetic population with observational data and projected onto the Galactic XY plane. The green circle denotes sources within $3.0\,\mathrm{kpc}$ of the Solar System, as listed in the catalog by 2013AA...558A..53K.
• Figure 4: (a) Modeled energy spectrum for accelerated nuclei up to charge $\mathrm{Z} = 40$, compared with recent experimental measurements from 2019arXiv190909073T2017PhRvD..96l2001A2021EPJC...81..966A2019AdSpR..64.2546G2017ICRC...35.1096M2008ApJ...678.1165A2020APh...11702406B. (b) Best-fit model parameters obtained from fitting the spectrum.
• Figure 5: Proton spectrum predicted by the model (solid line) compared with experimental measurements from 2009BRASP..73..564P2019PhRvL.122r1102A2017ApJ...839....5Y2019SciA....5.3793A2005APh....24....1A2013APh....47...54A2019AdSpR..64.2546G2008ApJ...678.1165A.