- Published on 25 October 2016
In section 2. Astrophysical processes
Cosmic-ray energy spectrum and composition up to the ankle: the case for a second Galactic component
A vital piece of the puzzle regarding the origin of Galactic cosmic rays comes from the two breaks in their power-law energy spectrum at 3 x 106 GeV (called the knee) and 3 x 109 GeV (the ankle). These breaks have remained a mystery for decades. It is clear that above about 109 GeV, the particles are not confined to their parent galaxies by interstellar magnetic fields and gas and, therefore, escape to produce a cosmic background. Below the knee, acceleration sites have been associated with supernova remnants and massive stellar wind shocks interacting with the interstellar medium account for the Galactic cosmic rays below a few 106 GeV; the primary cosmic rays are protons, He, and CNO. The models presented here confirm earlier results that other sources are required for the higher energy particles but goes substantially farther. The authors propose two new agents in the second Galactic component: the Galactic wind termination shock and the environment created by the Wolf-Rayet progenitor of core collapse supernovae whose initial composition depends on the stellar wind He/H abundances. Their models yield nuclear compositions at 108 GeV that include a significant contribution from Fe along with CNO, and none at around 109 GeV, while requiring a smaller extragalactic contribution below that energy than previous models. The paper presents an extensive set of model energy distributions for a broad range of nuclei yields (see Table 4) and a detailed comparison with existing measurements from 1-1011 GeV.