Galactic Constraints on the Sources of Ultra-High Energy Cosmic Rays
  We show that if the sources of ultra-high energy cosmic rays (UHECRs) with energies E>10^{19}eV are associated with galaxies, then the production of UHECRs must occur in transient events. Our galaxy is currently at a dim state in between transients. The time interval between transients is >3x10^4 years if the minimum Lorentz factor to which protons are accelerated in the transients is between 10^3 and 10^7. This constraint is satisfied by gamma-ray bursts. 
Abraham Loeb (Harvard), Eli Waxman (Weizmann)