Ultra High Energy Cosmic Ray Physics
The flux of cosmic ray nuclei has been relatively well-studied at energies below 1018 eV. To first approximation, the spectrum is a rapidly falling power law in energy, dN/dE ∝ E-α, with an overall index α of about 2.8. The spectrum does, however, show significant structure.
Structure in the cosmic ray spectrum
At the so-called "knee," a steepening of the spectrum at about 1015 eV, the spectral index α changes from about 2.7 to 3.0. A second steepening at about 5·1017eV (α=3.3) is followed by a harder spectral index (α=2.7) above 5·1018 eV.
Breaks in the cosmic ray spectrum are typically correlated with changes in the composition. In our current understanding, the decrease of flux in the knee region can be interpreted as a confinement problem: due to their increasing gyroradius, cosmic rays above a critical energy can more easily escape from the Galaxy.
As the gyroradius is proportional to the charge of the cosmic ray particle, this critical energy is smaller for lighter particles. With increasing energy, lighter nuclei will escape earlier, and consequently, the break in the spectrum at the "knee" is accompanied by a change from a mixed to a heavy composition.
Measurements of the composition around the "ankle" at 1018 eV also indicate a change of the cosmic ray composition towards a lower mean atomic number between 1017 and 1018 eV, with protons completely dominating the composition at the highest energies. The harder spectrum at 5·1018 eV is interpreted as the crossover from Galactic to extragalactic origin of the cosmic rays.
The GZK Effect and the Present Status of Experiment
We expect the cosmic ray spectrum to end around 6·1019 eV. This cutoff, first predicted by Greisen (1966) and Zatsepin and Kuz'min (1966) and named the GZK-cutoff, is expected due to the interaction of cosmic ray particles with the 2.7°K cosmic microwave background radiation. The collision of 1020 eV protons with 10-3 eV photons produces center of mass energies above 100 MeV, which is above the threshold for photo pion production. Subsequently, any proton or nucleus with a travel distance from its origin to the Earth of more than around 50 Mpc suffers severe energy losses, and independent of the original energy will end up with an energy below the GZK cutoff energy.
The AGASA cosmic ray experiment has found that the spectrum seems to continue beyond this energy without evidence for a cutoff. This leaves us with a two-fold problem: while it is already difficult to explain how ``traditional'' astrophysical sources can accelerate protons to energies above 1020 eV, the expected energy losses due to interaction with the microwave background require the sources to be relatively nearby, at a distance of 50 Mpc at most.
The situation is complicated by the fact that the deflection of protons in Galactic and intergalactic magnetic fields is less than a few degrees at these distances, so cosmic rays should point back to their origin. The distribution, however, seems uniform and shows no strong correlation with the matter distribution in the nearby universe.
Theoretical and Experimental Shortcomings
The mechanisms for accelerating particles to such incredible energies are presently unknown. Models can principally be divided into two classes: those seeking a more traditional approach in the framework of known astrophysical objects with powerful acceleration mechanisms (so-called "bottom-up" scenarios), and those circumventing the acceleration problem by directly creating particles of the highest energy ("top-down" models). Possible bottom-up sites include radio galaxies with powerful jets; the galaxy M87, at a distance of a mere 20 Mpc, is a prime suspect.
Unfortunately, current statistics are unconvincing. Whatever the sources of these energetic particles are, at this point, more data is needed. Several experiments currently operating (HiRes, AGASA), being built (Auger) or in the planning phase (Telescope Array, EUSO, OWL), will soon greatly enlarge and improve the existing data sample and bring us closer to the answer.
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