Cosmic ray production in superbubbles

Abstract

ABSTRACT We compute the production of cosmic rays (CRs) in the dynamical superbubble (SB) produced by a cluster of massive stars. Stellar winds, supernova remnants, and turbulence are found to accelerate particles so efficiently that the non-linear feedback of the particles must be taken into account in order to ensure that the energy balance is not violated. High-energy particles do not scatter efficiently on the turbulence and escape quickly after each supernova explosion, which makes both their intensity inside the bubble and injection in the interstellar medium intermittent. On the other hand, the stochastic acceleration of low-energy particles hardens the spectra at GeV energies. Because CRs damp the turbulence cascade, this hardening is less pronounced when non-linearities are taken into account. Nevertheless, spectra with hard components extending up to 1–10 GeV and normalized to an energy density of 1–100 eV cm−3 are found to be typical signatures of CRs produced in SBs. Efficient shock reacceleration within compact clusters is further shown to produce hard, slightly concave spectra, while the presence of a magnetized shell is shown to enhance the confinement of CRs in the bubble and therefore the collective plasma effects acting on them. We eventually estimate the overall contribution of SBs to the Galactic CR content and show typical gamma-ray spectra expected from hadronic interactions in SB shells. In both cases, a qualitative agreement with observations is obtained.