Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to whether cosmic particles of mean energy of 2 Joules are created in our Milky Way or in distant extragalactic objects.
Interesting article in the October 2016 issue of the American bi-weekly magazine ScienceNews.
Cosmic rays are mostly charged particles that reach Earth from outer space.
The Pierre Auger Collaboration has investigated the constraining power of the cosmic ray spectrum and composition measurements with respect to the properties of the possible sources.
The Pierre Auger Collaboration has searched for the possible presence of photons with energies exceeding 1018 eV in the flux of cosmic rays arriving at Earth.