An Irish researcher at the University of Leicester is part of a team thatbelieves it can explain gamma ray bursts, the most violent events in theuniverse, reports Dick Ahlstrom
Colossal isn't a big enough word to describe the discharge of energy released by a gamma ray burst. For a time, it emits more light than the billions of stars in an entire galaxy.
The source of such amazing outbursts has remained a mystery since the first gamma ray burst was detected in 1967. Now a team of scientists, led by astronomers from the University of Leicester, believes it has found the answer to this puzzle and has published its findings in a recent issue of the journal, Nature.
The second author of the paper is Dublin man Dr Darach Watson, who worked with noted researcher Prof Brian McBreen at University College Dublin. Watson explains how the discovery was made and the team's surprise at the findings.
A US military satellite first detected enormous discharges of gamma rays during the height of the Cold War. The satellite was meant to detect the radiation given off by nuclear warheads, and its operators were caught on the hop when a source was detected "behind" the satellite.
Thousands of similar bursts have been detected since, but the source was a mystery. The discharge typically lasts about 50 seconds, says Watson, but it could take hours to get a large land-based telescope pointed towards the source, far too late to see anything or to establish a cause. Concerted efforts since, by astronomers around the world, have improved our understanding.
Very few astronomical events deliver the punch of a gamma ray burst, explains Watson.
"The bursts themselves outshine the entire host galaxy where they occur. The energy they explode out is equivalent to the mass of the sun being converted directly to energy in a matter of seconds. It has been a major dilemma - how do you emit that much energy in such a short time?"
There were two competing models for what could release that amount of energy, he explains, the "hypernova" model and the "neutron star/black hole" model.
The first assumes that a young but unstable giant star, perhaps 50 to 100 times bigger than our own sun, burns out quickly and then blows up in an oversized supernova explosion, then collapses, probably within hours or days afterwards. This collapse causes the gamma ray burst.
The second assumes a collision between a pair of orbiting neutron stars or a neutron star and a black hole. Sometimes when a large star collapses a black hole is formed; other times a neutron star is formed. These are enormously dense objects with the mass of a star-sized body condensed into something no more than a few miles across. Two of these smashing together would give off a huge amount of energy.
The Leicester team used the European Space Agency's XXM X-ray telescope to study the X-ray "afterglow" that remains for days after a burst. Bursts are dominated by gamma rays but also give off other radiation signals from radio waves through visible light up to X-rays.
Watson was surprised to see clear information emerge when he started to look at the X-ray data: "That was not expected at all." He found clear "spectral" data about the matter that provided the source for the afterglow.
He saw lighter elements, silicon, sulphur, argon, calcium and a bit of nickel and magnesium. "These are basically hypernova products," he says, and are usually only seen when young stars die. Similar signals from a neutron star usually show heavier elements, particularly iron.
THE Leicester team believes this demonstrates a two-step process leading to a burst. First, a large unstable young star explodes, blasting away the lighter surface elements in an initial discharge and leaving behind a core of heavier elements. This material spreads out like a bubble in all directions, travelling at about one-tenth of the speed of light.
Next, something as yet unknown occurs in the core. "Something happens in the middle after a time, which we see as a gamma ray burst," says Watson. One theory, as yet unproven, holds that the excess angular momentum left behind in the smaller core tears it apart and that the pieces then collide to cause the burst.
The gamma rays coming from the burst travel out at the speed of light and soon overtake the bubble of lighter material, irradiating it and causing it to give off the X-rays later seen by the XMM. "There are a few things we can nail down very well from the X-ray observations," Watson says, including the size, speed and composition of the bubble and the time between the hypernova and the burst.
He and the team are confident that this description takes us well along the road towards understanding what causes a burst. "It is difficult to explain gamma ray bursts in any other way," he says, although he admits that the controversy is far from over.
Not all astronomers are convinced that this adequately explains a burst - so the research will continue.