Bright idea to detect space blasts

A simulation of the shape of the Cherenkov light splash

By BBC News Online science editor Dr David Whitehouse

The largest solar furnace ever built is to be modified to become one of the most unusual telescopes ever pointed at the heavens.

The complex has been a pilot for future energy technology

Solar Two, built in California's Mojave Desert, consists of more 1,800 mirrors that focus sunlight on to a laboratory placed on a tower in the middle of the array. Temperatures of thousands of degrees can be generated.

The complex, which generated 10 megawatts of power until 1998, is now being converted to detect gamma-rays, high-energy radiation from the depths of the cosmos. It is hoped that the array of mirrors will be able to detect the flash of light produced when gamma-rays strike our atmosphere.

Gamma-rays are of great interest to astrophysicists because they come from the violent parts of the Universe, offering clues about the birth of galaxies, the large-scale distribution of matter and the mysterious, outrageously violent events known as gamma-ray bursters.

Faster than light

When gamma-rays hit the upper atmosphere, they produce a shower of electron and anti-electron particles. Because the gamma-rays are so energetic, the particles in the air shower move faster than the speed of light in the atmosphere (which is less than the speed of light in a vacuum).

The result is that they emit so-called Cherenkov radiation in much the same way that an aeroplane moving faster than the speed of sound in air produces a sonic boom.

Big enough to contain the Cherenkov light splash

On the ground, the Cherenkov flash lasts for a billionth of a second and is in the form of a circle of light spread out over the size of a football field. It is very difficult to detect.

Solar Two, the largest solar farm in the world, is the only such complex capable of containing the entire Cherenkov light splash.

Stellar corpses

Gamma-ray astronomy has developed rapidly in recent years, thanks in part to the research undertaken with the Compton Gamma-Ray Observatory. The satellite, which was de-orbited in June last year after nine years of service, produced a wealth of data for scientists.

The mirrors reflect the Cherenkov light pool on to the central tower

One of the great puzzles astronomers would like to solve concerns an event called a gamma-ray burster. This is a colossal flash of radiation which can, within a few seconds or minutes, match the energy our Sun will produce in its entire lifetime.

What causes the flash remains a mystery, although recent research suggests the sudden collapse of stellar corpses - neutron stars - to form black holes may hold the key to at least some of the blasts.

The Compton was used to probe the bursts but could only measure gamma-rays of moderate energies. Solar Two, however, could measure those with 20 times as much energy, possibly solving the puzzle.

Solar Two has already detected some gamma-rays from the Crab Nebula. This wreckage from an exploded star is a well-known source that will help calibrate the telescope when its conversion is complete.