Massive spectacle of gamma ray burst all over in 10 seconds

Imagine taking all the energy produced by the Sun over its five-billion-year lifetime and discharging it in one massive 10-second blast. The resultant explosion would only just match the ferocity of one of the most massive spectacles in the universe, which astrophysicists are only now beginning to understand - gamma ray bursts.

They are by far the single most luminous objects in the universe during the few seconds that they exist. A gamma ray burst occurring more than a thousand billion miles away from earth would still appear a hundred times brighter in the sky than the sun.

These bursts, detected by satellites and seen as a sudden incoming wave of gamma ray radiation, were first detected in the late 1960s. US military experts found them while watching for Russian infringements of the nuclear test ban treaty and because of military secrecy waited until 1973 to report the phenomenon publicly.

Scientists still don't know what they are however, said Dr Lorraine Hanlon of the department of experimental physics at NUI Dublin. "That is the big unknown. They seem to be some kind of fireball."

Dr Hanlon is co-ordinator of a European consortium involving Dutch, Italian and Spanish specialists who hope to learn more about these elusive explosions. Satellites detect about one burst each day, she said, but because they are so short-lived - between one and 100 seconds, with an average lifespan of 10 seconds - they are difficult to locate while the event is occurring. Localising the event is important because astronomers surmised there must be some visible light coming from a burst. If the remnants of a burst could be seen then its light could be analysed to determine how far away the burst occurred. Bursts had been localised using triangulation but the method was slow and when astronomers went to look using optical telescopes, there was nothing left to see. All this changed, however, with the May 1996 launch of the Italian BeppoSax satellite which detects X-ray and gamma ray radiation. It can scan over a wide field but can then be focused down to a narrow field if gammas are detected. It recorded its first burst in August 1996 but it was not until February 28th, 1997, that astronomers caught their first glimpse of a burst as it died away. BeppoSax localised incoming gamma rays and optical telescopes around the world focused in on the spot.

All they got was a smudge of light, stated Dr Hanlon, but that was enough to show that visible light was emitted and also that a burst could be caught in the act. Then on May 8th, 1997 astronomers obtained a spectrum of the light coming from a burst allowing its infra-red "shift" to be read and its distance from earth to be calculated.

The consortium of researchers now use an array of optical telescopes to catch remnant light from bursts, alerting operators when BeppoSax locates a fireball. BeppoSax detected bursts on December 27th and January 9th, but was not pointed in a way that allowed it to identify their exact locations. The consortium is hoping for several more burst locations so the InfraRed Space Observatory (ISO) satellite can read them, but time is running out. ISO uses liquid helium to keep its infra-red detectors cold but its supply will finally boil away sometime in April. Only one light spectra from a burst has been captured and many more will be needed before researchers can understand what these massive events are. They give off the energy of a supernova, when ageing stars explode, but the blast occurs not over years, but over seconds.

"The energy requirement is gigantic. We still don't know what would give that amount of energy in that amount of time, " Dr Hanlon said. "It is going to be something cataclysmic."

Scientists are unsure what happens during a gamma ray burst but they can detect electromagnetic radiation coming from it. Intense heat at the point of explosion causes atoms to break apart into ions, creating a massive plasma at the core of the burst. Energetic gamma ray radiation is first able to break out of the plasma and this radiation gives the event its name. As the plasma ages it is less able to trap in radiation and eventually X-rays, visible light and radio waves escape from the explosion. Scientists can read the incoming radiation to help determine how far away the burst occurred.