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

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.