Supernova captured in 'real time'

From: http://news.bbc.co.uk/2/hi/science/nature/5298726.stm

Aug 29, 2006

The shockwave bursts through the star's surface, releasing its energy

Astronomers say they have witnessed a stellar explosion - or supernova - unfolding in real time.

Supernovae occur when huge, mature stars effectively run out of fuel and collapse in on themselves.

The observations, outlined in the journal Nature, offer the most detailed picture yet of these cosmic explosions.

An initial release of energy from the star was picked up by the Swift satellite in February, allowing experts to train their telescopes on the event.

This short, sharp outburst, known as an X-ray flash, amounted to an early warning signal that the star was going to turn supernova.

Energy release

Swift is a US-British-Italian mission designed to look out for gamma-ray bursts (GRBs), intense and energetic outbursts of radiation thought to arise from the collapse of massive stars in a supernova.

However, not all supernovae generate GRBs, and it remains to be determined what causes some but not others to produce these bursts.

The stellar explosion occurred 400 million-light years away

An X-ray flash can be regarded as a less energetic form of GRB. It was previously unclear whether these lower-energy events were generated by the same process of stellar collapse during a supernova. The latest results seem to confirm that link.

The release of energy that occurs in the X-ray flash lasts for about 15 minutes and is followed by a day-long "breakout" phase, in which a shockwave is thought to breach the surface of the collapsing star, unleashing its energy.

This expands into space as radiation at all frequencies and lasts for a period of days to months - the classic signature of a supernova.

Four groups of astronomers gathered data on the event, and have now published separate research papers in Thursday's issue of Nature.

Compact remnant

Mission team member Paul O'Brien, of the University of Leicester, UK, said: "The three onboard telescopes detected a slowly brightening then fading object.

"The results suggest a broad jet expanded into the surroundings but it was accompanied by a slower-moving and incredibly hot - two-million-degree - bubble of gas produced from the shockwave of the exploding star."

Analysis of the expanding debris suggests the initial star had a lower mass than is the norm for such explosions. Gamma-ray bursts are thought to be associated with the collapse of a star into a black hole. But lower mass stars are believed to collapse into a compact object known as a neutron star.

"Those observations capture the time when the star collapsed, ejected its outer envelope and most of its mass and left a compact remnant behind. And we think that compact remnant is a neutron star," said Paolo Mazzali, one of the authors who is based at the Max Planck Institute for Astrophysics in Garching, Germany.

So what powered the X-ray flash and shockwave? The researchers think these were produced by energy supplied by the highly magnetised neutron core, or magnetar as it is known.