High energy protons produced by various sources of cosmic rays, { \it e.g.} supernova remnants, pulsar wind nebulae, active galactic nuclei and gamma-ray bursts participate in $p\gamma$ and $pp$ interactions. Although $pp$ interactions may be the dominant mechanism in our Galaxy, it is unclear how important $p\gamma$ process is. We show that the upper bound on the fraction of protons participating in $p\gamma$ interactions inside all Galactic astrophysical sources of cosmic rays is 10%.
Categories: astro-ph hep-ph