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Ultra High Energy Cosmic Rays (UHECR)

The origin of the highest energy cosmic rays is one of the outstanding puzzles in astrophysics. These particles (mostly protons, but including heavier atomic nuclei) have energies of up to around 1020 eV (16 Joules), over fifty million times more energetic than the particles produced by the Fermilab Tevatron accelerator. We can only speculate about the conditions at the their cosmic source, because we know of no standard object (supernova, pulsar, or even a black hole) that could easily accelerate particles to such enormous energies.

One possibility is that the highest energy cosmic rays are accelerated in the giant relativistic jets emitted from the centres of active galaxies, like those shown in the picture. It is believed that these jets are powered by the super-massive black hole at the nucleus of the galaxy. Cosmic rays might gain macroscopic energy through interactions with magnetic shocks in the fast moving jets. But this is speculation, as we have not yet observed cosmic rays coming directly from such objects.

The search for the sources of cosmic rays has been hampered by by their rareness. At the highest energies, the particles reach the top of the Earth's atmosphere at rates ranging from 1 per square meter per century above 1017 eV to 1 per square kilometre per century above 1020 eV! In order to collect a significant number of events, large area detectors are required.

Fortunately, Nature has helped here with what is known as an extensive air shower (EAS). This is a cascade of millions of subatomic particles initiated when a single high energy cosmic ray collides with an air molecule high in the atmosphere. The energy of the initial cosmic ray is converted into matter (E=mc2), with the cascade growing and then declining as it moves further into the atmosphere. Extensive air showers started by cosmic rays with even relatively modest energies of 1014 eV will survive all the way down to ground level. Over the years, many experiments have been mounted to detect these EAS. Typically, such a detector consists of a large number of particle detectors (e.g. scintillators) spread over an area on the ground. An EAS hitting the ground looks like a large pancake of particles travelling at the speed of light, with a diameter of up to a kilometre and a thickness of a few metres. The existence of EAS has made the study of the highest energy cosmic rays possible, but it is still a very difficult task to collect sufficient numbers of them.

Fluorescence detectors like the Fly's Eye and its successor HiRes, view light initiated by the EAS in the Earth's atmosphere. They can view this light from large distances, and therefore accept showers over a huge collecting area. The Auger Project is also using fluorescence detectors, but is also coupling them with a giant array of particle detectors covering 3000 square kilometres.

Another difficulty in determining the origin of these particles is the presence in our Galaxy of a weak, but ever present, magnetic field. Weaker magnetic fields also exist between galaxies. Being mostly charged particles, cosmic rays are deflected by magnetic fields. This is less of a problem when the particle energy gets really large, when they are expected to slice through magnetic fields with deflections of only a degree or two, but at lower energies the arrival directions carry no information on the direction of the cosmic ray source. So looking at the highest energy cosmic rays is sensible if you want to trace their source directions - its just a pity that the flux is so low! However we believe that at all energies a small fraction of the cosmic ray beam is made up of neutral particles, in particular gamma-rays and neutrons, which would travel in a straight line from their sources. It is a goal of most experiments to try to view these neutral particle directions in amongst the more numerous charged cosmic rays.

Giant Jets from an Active Galaxy
Giant relativistic jets emitted from the centre of an active galaxy. Click on the image to view a large version [432x246, 53KB, JPEG].

Energy Spectrum
Energy spectrum showing the frequency of events at different energies. Click on the image to view a large version [518x643, 9KB, GIF].

Extensive Air Shower
A diagram of the propogation of an extensive air shower showing the particles produced. Click on the image to view a large version [612x608, 63KB, GIF].