We show that the secondary electrons ejected from the ionization of heavy ions can be injected into the acceleration process that occurs at supernova remnant shocks. This electron injection mechanism works since ions are ionized during the acceleration when they move already with relativistic speed, just like ejected electrons do. Using the abundances of heavy nuclei measured in cosmic rays measured at the Earth, we estimate the $\mathrm{\text{electron}}/\mathrm{\text{proton}}$ ratio at the source to be $\sim {10}^{-4}$, big enough to account for the nonthermal synchrotron emission observed in young supernova remnants. We also show that the ionization process can limit the maximum energy that heavy ions can reach.

• Received 11 May 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.121102