BioEd Online

A source of ultra-high-energy cosmic rays has been identified for the first time. The extremely energetic particles, racing at almost the speed of light, have been traced back to a pair of colliding galaxy clusters.

Cosmic rays are fast-moving particles that constantly bombard the Earth. Some come from the Sun, whereas higher-energy rays are accelerated around the remnants of supernovae.

But ultra-high-energy cosmic rays are at least a thousand times more energetic still, and extremely rare. Only one particle is expected to hit each square kilometre of Earth every century.

Physicists already know that these particles are almost certainly protons whose energies are measured in exaelectronvolts (1018 eV) - the amount of energy that an electron acquires when it is accelerated by a billion billion volts. Each proton has a kinetic energy similar to that of a flying golfball, and travels at just one part in 1022 slower than the speed of light.

Glennys Farrar, a particle physicist from New York University, has now shown that five of these particles all came to Earth from a pair of galactic clusters that are crashing together roughly 450 million light years away.

Getting it straight

Only about 100 of these ultra-high-energy cosmic rays have been detected on Earth in the past decade, and until now all of them seemed to come from different parts of the sky. They travel so fast that magnetic fields in space can barely alter their paths.

This means that the particles travel in a virtually straight line to Earth, allowing Farrar to plot their paths and work out where they came from.

Her data came from two separate detectors. The High Resolution Fly's Eye fluorescence detector in Dugway, Utah, searches Earth's atmosphere for streaks of light generated when the particles hit. "The impacts make the atmosphere fluoresce in a line," explains Farrar, and that line points directly at the source.

The impacts also start a cascade of secondary reactions in the air, generating millions more particles that shower Earth's surface. The Akeno Giant Air Shower Array (AGASA) in Asao, Japan, has particle detectors covering an area of 100 km2.

If all the detectors fire at the same time, this indicates a massive event that could only have been triggered by the arrival of an ultra-high-energy cosmic ray.

Line of sight

The five events that Farrar tracked were spotted between 1993 and 2003. The difference in their arrival time is due to the very slightly different paths they took to reach Earth even the tiniest deviation from a straight line can make a decade's difference over such huge distances.

The paths all point to the merging galaxy clusters, Farrar calculated. Her conclusion is supported by observations from the Sloan Digital Sky Survey, run from the Apache Point Observatory in New Mexico, which show that there is a clear line of sight between Earth and the clusters.

"It's the combination of three different sets of data that makes this finding compelling and believable to me," says Lynn Cominsky, a physicist from Sonoma State University, California.

Field effect

The clusters are extremely rich in stars and have powerful magnetic fields that become warped when they collide. It's possible that the turbulent magnetic fields accelerate charged particles such as protons in tight spirals before flinging them towards us.

Some physicists have speculated that such energetic particles could only come from the decay of exotic, heavy subatomic particles formed immediately after the Big Bang. But seeing five high-energy rays from the same point rules that out, says Farrar, because the chance of finding that many exotic decays along the same line of sight is minuscule.

Farrar presented her results on 12 January at the American Astronomical Society conference in San Diego, California. She now hopes to dig deeper into existing data to find more potential sources for the particles, which should help her to understand the cataclysmic events that create them.