The Very Highest Energy Cosmic Rays

The very highest energy cosmic rays are of particular interest for various reasons. They may provide a useful tool for finding the origin of cosmic rays because they are deflected very little by the galactic and interstellar magnetic fields that permeate space. Therefore the direction in which they are travelling when they arrive at Earth should point back to the area of space were they came from.

There are many unanswered questions regarding their production:-

How are they produced?

Mechanisms to accelerate particles up to energies of 1015 eV have been proposed and generally consist of binary star systems (two stars in orbit around each other) or supernova remnants (the turbulent shell of gas left behind after a star has exploded). However the acceleration mechanisms involved in producing the highest energy cosmic rays are still unknown. There may even be new physics involved. One possibility is that they are generated by very massive particles produced at the beginning of time.

Where are they produced?

The places in the universe where cosmic rays of >1018 eV are produced must either have very large magnetic fields or be of enormous size. If the highest energy cosmic rays come from within our galaxy the production sites would be expected to be relatively close to Earth because the galaxy cannot trap such energetic particles within its magnetic field (unlike lower energy cosmic rays) and they would readily escape after travelling a short distance. If this were the case, we would expect to see more high energy cosmic rays coming from the direction of the galactic plane than from elsewhere.

However, if the highest energy cosmic rays come from outside our own galaxy they would not be able to travel for more than about 150 million light years. This may sound like a long way but it is only the distance to some of our neighbouring galaxies. The reason the high energy cosmic rays can not travel further is that as they race through space they occasionally bump into photons of microwave radiation. This radiation is the light left over from the big-bang explosion that created the universe. When the high energy cosmic rays hit microwave photons, they lose some of their energy. This effect is known as the "GKZ cut-off" and because of this many physicists believed no cosmic rays with energies above about 4x1019 eV existed. However experiments such as that at Haverah Park the Fly's Eye(USA) and AGASA(Japan) have shown that particles above the GKZ cut-off do exist.

Hubble Space Telescope image of Active Galactic Nuclei NGC-4261
Do the highest energy Cosmic Rays come from objects such as this? On the left is a radio telescope image of the active galaxy NGC-4261. The width of the image covers a region of space 88,000 light years across. To the right is a close up image from the Hubble Space Telescope covering just 400 light years. It shows a doughnut shaped ring at the centre. This ring is thought to orbit a giant black hole with a mass more than a hundred million times that of our sun.

What is the highest energy cosmic ray ever detected?

In 1993 the "Fly's Eye" experiment in Utah detected a cosmic ray with an energy of 3x1020 eV. So far this is the highest energy particle ever detected. This particle had a kinetic energy similar to that carried by a tennis ball travelling at 180 mph! Cosmic Rays are 1014 times smaller than tennis balls so the energy is packed into an incredibly small volume.

How can we study the highest energy cosmic rays?

Cosmic rays with energies above 1020 eV are very rare. On average one such particle hits each square kilometre of the Earth only once a century. So to detect a large number of them and study them in detail we need a huge detector. The Pierre Auger Observatory will eventually consist of two observatories, one in the northern hemisphere and one in the south. Each will have 1600 water Cerenkov detectors spread over 3000 km2 and 3 fluorescence light detectors. The first part of the observatory will be built in Argentina. Construction commenced on March 17th 1999. When this observatory is completed sometime in the next century we may finally be able to answer the question. "Where do cosmic rays come from?".