Extragalactic cosmic ray

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Extragalactic cosmic rays are very-high-energy particles that flow into our solar system from beyond our galaxy. The energies these particles possess are in excess of 10¹⁵ eV.

Contrary to Solar cosmic rays or galactic cosmic rays, little is known about the origins of extragalactic cosmic rays. Partially, this can be attributed to a lack of statistics: the amount of cosmic rays reaching the Earth's surface originating from extragalactic sources, is about 1 particle per square meter per year; see figure.

[edit] Origins

There are many ideas about which processes may be responsible for cosmic rays with such high energies. In the 'bottom-up' approach, cosmic rays gain more and more energy through electromagnetic processes. Bouncing randomly back and forth in the shock waves of some violent object allows some of the particles gain energy. Eventually they may build up enough speed that the remnant can no longer contain them, and they escape. Possible sites typically include gamma ray bursts and active galactic nuclei^[1].

There are many more possible sources of cosmic rays scientists are considering. These include nearby galaxies, colliding galaxy systems, accretion flow shocks to clusters of galaxies, and more exotic processes from the very early universe, such as the decay of superheavy particles trapped in the galactic halo, or topological defects^[2].

[edit] References

1. ↑  Fonseca, M.V., Very High Energy Cosmic Rays, Nuclear Physics B (Proc. Suppl.) 114 (2003), pp. 233-246
2. ↑  Nagano, M. and Watson, A.A., Observations and implications of the ultrahigh-energy cosmic rays, Reviews of Modern Physics 72 (2000) pp. 689-732