Cosmic rays from massive star clusters: a close look at Westerlund 1

We study the effect of cosmic ray (CR) acceleration in the massive compact star cluster Westerlund 1 in light of its recent detection in gamma-rays. Recent observations reveal a 1/r radial distribution of the CR energy density. Here, we theoretically investigate whether or not this profile can help to distinguish between (1) continuous CR acceleration in the star cluster stellar wind-driven shocks and (2) discrete CR acceleration in multiple supernovae (SNe) shocks - which are often debated in the literature. Using idealized two-fluid simulations and exploring different acceleration sites and diffusion coefficients, we obtain the CR energy density profile and luminosity to find the best match for the gamma-ray observations. We find that the inferred CR energy density profiles from observations of gamma-ray luminosity and mass can be much different from the true radial profile. CR acceleration at either the cluster core region or the wind termination shock can explain the observations, if the diffusion coefficient kappa cr similar to 10(27 )cm(2) s(-1) and a fraction of approximate to 10 - 20 per cent of the shock power/post-shock pressure is deposited into the CR component. We also study the possibility of discrete SNe explosions being responsible for CR acceleration and find that with an injection rate of 1 SN in every similar to 0.03 Myr, one can explain the observed gamma-ray profile. This multiple SN scenario is consistent with X-ray observations only if the thermal conductivity is close to the Spitzer value.

Automated summary

We investigated the effect of cosmic ray acceleration in the star cluster Westerlund 1 and found that the observed energy density profile of cosmic rays can be different from the true radial profile. We explored continuous and discrete acceleration scenarios and found that both can explain the observed gamma-ray profile.