White Paper on Ultra-High Energy Cosmic Rays
  A fundamental question that can be answered in the next decade is: WHAT IS
THE ORIGIN OF THE HIGHEST ENERGY COSMIC PARTICLES? The discovery of the sources
of the highest energy cosmic rays will reveal the workings of the most
energetic astrophysical environments in the recent universe. Candidate sources
range from the birth of compact objects to explosions related to gamma-ray
bursts or generated around supermassive black holes in active galactic nuclei.
In addition to beginning a new era of high-energy astrophysics, the study of
ultra-high energy cosmic rays will constrain the structure of the Galactic and
extragalactic magnetic fields. The propagation of these particles from source
to Earth also probes the cosmic background radiation and gives insight into
particle interactions at orders of magnitude higher energy than can be achieved
in terrestrial laboratories. Next generation observatories designed to study
the highest energy cosmic rays will have unprecedented sensitivity to
ultra-high energy photons and neutrinos, which will further illuminate the
workings of the universe at the most extreme energies. For this challenge to be
met during the 2010-2020 decade, a significant increase in the integrated
exposure to cosmic rays above 6 1019 eV will be necessary. The technical
capabilities for answering this open question are at hand and the time is ripe
for exploring Charged Particle Astronomy.
