Cygnus OB2 as a test case for particle acceleration in young massive star clusters

TL;DR

This study investigates whether cosmic ray acceleration at the wind termination shock of Cygnus OB2 can explain observed gamma-ray emissions, highlighting the importance of magnetic turbulence properties in modeling high-energy astrophysical phenomena.

Contribution

The paper presents a model linking cosmic ray acceleration at star cluster wind shocks to gamma-ray observations, emphasizing the role of turbulence spectra in emission characteristics.

Findings

Model fits VHE gamma-ray data with Kraichnan or Bohm turbulence.

The scenario is incompatible with Kolmogorov turbulence.

Additional effects are needed to explain Fermi-LAT morphology.

Abstract

In this paper, we focus on the scientific case of Cygnus OB2, a northern sky young massive stellar cluster (YMSC) located towards the Cygnus X star-forming complex. We consider a model that assumes cosmic ray acceleration occurring only at the termination shock of the collective wind of the YMSC and address the question of whether, and under what hypotheses, hadronic emission by the accelerated particles can account for the observations of Cygnus OB2 obtained by Fermi-LAT, HAWC and LHAASO. In order to do so, we carefully review the available information on this source, also confronting different estimates of the relevant parameters with ad hoc developed simulations. Once other model parameters are fixed, the spectral and spatial properties of the emission are found to be very sensitive to the unknown properties of the turbulent magnetic field. Comparison with the data shows that our suggested scenario is incompatible with Kolmogorov turbulence. Assuming Kraichnan or Bohm type turbulence spectra, the model accounts well for the Very High Energy (VHE) data, but fails to reproduce the centrally peaked morphology observed by Fermi-LAT, suggesting that additional effects might be important for lower energy $\gamma$-ray emission. We discuss how additional progress can be made with a more detailed and extended knowledge of the spectral and morphological properties of the emission.