20 - The origin of high energy protons and nuclei

Summary

γ-ray observations of the Galaxy

Just as the Galactic radio emission outlines the distribution of high energy electrons and magnetic fields in the Galaxy, so the distribution of γ-radiation can provide information about high energy protons and the overall distribution of interstellar gas. As described in Section 5.4, in collisions between high energy particles and protons and nuclei of atoms and molecules of the interstellar gas, pions of all charges, π⁺, π⁰ and π^-, are produced. The positive and negative pions decay into positive and negative muons, which, in turn, decay into positrons and electrons with relativistic energies (see Fig. 5.11). The latter may make a contribution to the low energy electron spectrum, and the predicted presence of positrons provides a direct test of the importance of the pion production mechanism in interstellar space. The neutral pions decay almost instantly into two γ-rays. In proton-proton collisions, the cross-section for the production of a pair of high energy γ-rays is roughly the geometric size of the proton, σ_γ ≈ 10^-30 m². The spectrum of γ-rays produced in such collisions is shown in Fig. 20.1. The characteristic signature of this process is that the spectrum of γ-rays has a broad maximum at about 70 MeV. Knowing the physical conditions in the interstellar gas, the γ-ray production rates by various mechanisms can be worked out. Stecker (1977) has carried out these calculations assuming an interstellar gas density of 10⁶ particles m^-3 and a local energy density of starlight of 4.4 × 10⁵ eV m^-3.