Extragalactic cosmic ray

The exact energy at which the transition from galactic to extragalactic cosmic rays occurs is not clear, but it is in the range 1017 to 1018 eV.

[1] The observation of extragalactic cosmic rays requires detectors with an extremely large surface area, due to the very limited flux.

As a result, extragalactic cosmic rays are generally detected with ground-based observatories, by means of the extensive air showers they create.

In either case, the ultimate aim is to find the mass and energy of the primary cosmic ray which created the shower.

This hybrid methodology allows for a full three-dimensional reconstruction of the air shower, and gives much better directional information as well as more accurate determination of the type and energy of the primary cosmic ray than either technique on its own.

One of the Pierre Auger Observatory's most notable results is the detection of a dipole anisotropy in the arrival directions of cosmic rays with energy greater than 8 x 1018 eV, which was the first conclusive indication of their extragalactic origin.

[15][16] More recently the Pierre Auger Observatory also observed a steepening of the cosmic ray spectrum above the ankle,[17] before the steep cutoff above than 1019 eV (see figure).

[19] It is unclear whether this is the result of an unknown systematic error or a true difference between the cosmic rays arriving at the Northern and Southern hemispheres.

This is largely due to a lack of statistics: only about 1 extragalactic cosmic ray particle per square kilometer per year reaches the Earth's surface (see figure).

Active galactic nuclei (AGNs) are well known to be some of the most energetic objects in the universe, and are therefore often considered as candidates for the production of extragalactic cosmic rays.

[32] Studies have found that shocks in clusters can accelerate iron nuclei to 1020 eV,[33] which is nearly as much as the most energetic cosmic rays observed by the Pierre Auger Observatory.

[18] However, if clusters do accelerate protons or nuclei to such high energies, they should also produce gamma ray emission due to the interaction of the high-energy particles with the intracluster medium.

[42] The photodisintegration of the heavy nucleii would produce lighter elements with lower energies, matching the observations of the Pierre Auger Observatory.