The population of Galactic young massive star clusters in the TeV range

Abstract

Young massive star clusters (YMSCs) can produce gamma rays in the very-high-energy (VHE, E>100 GeV) range and have been proposed as sources that can accelerate cosmic rays up to PeV energies. Observations with current instruments have lead to the detection of only a few YMSCs but future instruments should significantly increase this number. However, the details of the production of the VHE emission are not well understood: What is the spectrum of accelerated particles? What is the efficiency of cosmic-ray production? What fraction of the wind luminosity is converted into the turbulent magnetic field? To address these questions, we simulate the population of YMSCs in the gamma-ray domain, by means of Monte Carlo methods, and apply the constraints based on the subsample of YMSCs currently detected at TeV energies. We confront our simulated populations with the catalogue of the H.E.S.S. Galactic Plane Survey and the First LHAASO Catalogue of Gamma-Ray Sources, allowing us to investigate crucial aspects of particle acceleration at YMSCs.

Figures (3)

• Figure 1: Example of one realisation of a simulation of the population of Galactic YMSCs. The grey shaded region is the HGPS detectability range for point like sources with a luminosity of $5\times 10^{33}$ photons $\text{s}^{-1}$ (equivalent to a differential flux at 1 TeV of $\sim 4 \times 10^{-11} \rm TeV^{-1} cm^{-2} s^{-1}$ at a distance of 1 kpc). The turquoise shaded region is the LHAASO detectability range for point like sources with a luminosity of $1\times 10^{32}$ photons $\text{s}^{-1}$ and $3\times 10^{27}$ photons $\text{s}^{-1}$ (equivalent to a differential flux of $\sim 3 \times 10^{-13} \rm TeV^{-1} cm^{-2} s^{-1}$ and $\sim 5 \times 10^{-19} \rm TeV^{-1} cm^{-2} s^{-1}$ at 3 TeV and 50 TeV respectively at a distance of 1 kpc) for the WCDA and KM2A respectively. The black dots mark the simulated sources which are not detectable by any of the instruments, the blue plusses mark the simulated sources only detectable by H.E.S.S., the green squares mark sources only detectable by the KM2A, the red triangles mark sources detectable by both LHAASO instruments and the pink diamonds mark sources detectable by all the instruments. 26% of the YMSCs are detectable, by at least one instrument, in this realisation. The Sun is marked by the red cross and the spiral arms of the Milky Way are shown in black.
• Figure 2: 2D histogram plots showing the percentage of populations are in agreement with the observational data, i.e. have at least 2 detectable YMSCs and are within the upper and lower limits for all three detectors (H.E.S.S., WCDA and KM2A). These populations use the Kraichnan diffusion regime. The x-axis shows the fraction of the wind luminosity converted into cosmic rays, the y-axis shows the fraction of the wind luminosity converted into magnetic field. Moving from left to right shows a hardening spectral index.
• Figure 3: Same as Fig. \ref{['fig:Kraichnan']} except with Bohm diffusion.