The Highest-Energy Cosmic Rays Cannot be Dominantly Protons from Steady Sources

Understand

The bulk of observed ultrahigh energy cosmic rays could be light or heavier elements, and originate from an either steady or transient population of sources.

• This leaves us with four general categories of sources.
• Energetic requirements set a lower limit on single source luminosities, while the distribution of particle arrival directions in the sky sets a lower limit on the source number density.
• The latter constraint depends on the angular smearing in the skymap due to the magnetic deflections of the charged particles during their propagation from the source to the Earth.