Australasian Science: Australia's authority on science since 1938

The Biggest Bang

By Stephen Luntz

An extraordinarily long explosion has caused a rethink of the nature of gamma ray bursts.

Supernovae, produced when large stars collapse, were once thought to be the most dramatic events in the universe. Then satellites detected signals of enormously powerful gamma ray bursts from distant galaxies, followed by an afterglow in the visible light. These bursts typically last for 20–50 seconds, with a subclass shorter than 2 seconds.

However, when observations taken from three different satellites on 9 December 2011 were compared it was realised that a single burst had lasted for 7 hours. A/Prof David Coward of the University of Western Australia’s School of Physics has explained how such an event could occur in The Astrophysical Journal.

“We looked at various models, for example a highly magnetised neutron star, but the problem is that it would break up over that period of time,” says Coward. “The only thing we can think of is a highly massive star that collapses to form a black hole but also creates an unstable accretion disk that takes some time to fall onto the hole, extending the length of the burst.” The energy from the mass falling on the hole is funnelled into narrow jets, one of which passes Earth.

For this to occur “requires special conditions,” Coward says. He thinks this involves a supergiant blue star with very low metal content.

A separate team reached similar conclusions on the explosion’s nature, but thinks the progenitor star may have been a red giant with more metal. The event aroused so much interest that NASA ran a symposium for the two teams.

Coward says the energy emitted per second was lower than average for gamma ray bursts, but was “not an outlier”, making the total energy hundreds of times larger than what had been thought to be the most energetic event in the universe. Two other bursts have been observed that bridge the gap between the 2011 spectacular and ordinary gamma ray bursts, suggesting the existence of a subclass.

One even longer event has been detected, but Coward says this has completely different characteristics and appears to be the result of the supermassive black hole at the centre of a galaxy swallowing a star.

The event occurred in a galaxy with a redshift of 0.6, or 5.5 billion light years away, which is far too distant for the progenitor star to be visible in prior images.