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The University of Southampton

Brightest explosion ever observed overwhelms telescopes

Published: 18 February 2005

Scientists from Southampton are among those who have detected a flash of light from across the Galaxy so powerful that it bounced off the Moon and lit up the Earth's upper atmosphere. This "giant flare" was the brightest explosion ever detected from beyond the Solar System. For over a tenth of a second the remarkable flare was actually brighter than a full moon.

NASA and European satellites and ground-based telescopes around the world detected the giant flare on 27 December 2004. Scientists from twenty institutes joined the observations. Two science teams, one of them from Southampton, report about this unprecedented event in a forthcoming issue of Nature.

The light detected from the giant flare was far brighter in gamma rays than visible light or X-rays. It was probably created by an unprecedented eruption on the surface of an exotic neutron star, which is classed both as an ultra-magnetic magnetar and as a soft gamma repeater (SGR). The designation of the neutron star that erupted is SGR 1806-20, about 50,000 light years from Earth in the constellation Sagittarius.

A number of important questions arise from this discovery:
* Are some gamma ray bursts (thought to be very distant black-hole-forming star explosions) actually from neutron star eruptions in nearby galaxies?
* What mechanism could unleash so much energy from a magnetar?
* Could an even larger influx of gamma rays have caused mass extinction on Earth in the past?

British astronomers have focused on studying the radio emission from the event, which was produced as the explosion ploughed into the surrounding matter at about 100,000 km per second, heating particles to extraordinary energies. Dr Rob Fender of the University of Southampton's School of Physics and Astronomy is co-author on a Nature paper describing the radio observations.

"This is a once-in-a-lifetime event. We have observed an object only 20 kilometres across, on the other side of our Galaxy, releasing more energy in a tenth of a second than the Sun emits in 100,000 years," said Dr Fender.

"The next biggest flare ever seen from any soft gamma repeater was peanuts compared to this incredible December 27 event," said Dr Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics, lead author on the Nature paper. "Had this happened within 10 light years of us, it would have severely damaged our atmosphere and possibly have triggered a mass extinction. Fortunately there are no magnetars anywhere near us."

"These neutron stars have magnetic fields hundreds of times more powerful than any other objects in the universe. We may be seeing a massive release of magnetic energy during a 'starquake' on the surface of the object," added Dr Maura McLaughlin of the University of Manchester, also a co-author on the Nature paper.

Radio observations of the neutron star continue around the world, including the UK's Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Joint Institute for VLBI in Europe.

Notes for editors

A neutron star is the remnant of a star that was once several times more massive than our Sun. When such stars deplete their nuclear fuel, they explode - an event called a supernova. The remaining dense core is slightly more massive than the Sun but has a diameter of only 20 km. It is also highly magnetic and has an extremely rapid rotation.

SGR 1806-20 spins once in only 7.5 seconds. Amazingly, the December 27 event did not cause any slowing of its spin rate, as would be expected.

Millions of neutron stars fill our Milky Way galaxy. However, scientists have discovered only a dozen or so ultra-magnetic neutron stars, called magnetars. The magnetic field around a magnetar is about 1,000 trillion gauss, strong enough to strip information from a credit card at a distance halfway to the Moon. (Ordinary neutron stars measure about a trillion gauss; the Earth's magnetic field is about 0.5 gauss.)

Four of these magnetars are also called soft gamma repeaters, or SGRs, because they flare up randomly and release gamma rays. Such episodes release huge amounts of energy - the giant flare on SGR 1806-20 unleashed about 10,000 trillion trillion trillion watts.

The tremendous magnetic fields around a magnetar are responsible for SGR outbursts, but scientists do not understand the details. Perhaps an outburst is caused by magnetic reconnections, like an ultra-powerful solar flare, or a quake on the surface of the neutron star.

A scientific debate raged in the 1980s over whether gamma ray bursts were star explosions from beyond our Galaxy or eruptions on nearby neutron stars. By the late 1990s it became clear that gamma ray bursts did indeed originate very far away and that soft gamma repeaters were a different phenomenon.

However, the extraordinary giant flare on SGR 1806-20 opens up new possibilities, according to Dr. Chryssa Kouveliotou of NASA's Marshall Space Flight Center, who took a central role in co-ordinating the global observations.

"A small percentage of "short" gamma ray bursts that last less than two seconds could be SGR flares," she said. (Long gamma ray bursts appear to be black-hole-forming star explosions billions of light years away.)

Image: the highly magnetised star (courtesy of NASA)

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