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The night the sky exploded
18/02/2005 21:44 - (SA)
Richard Ingham
Paris - Stunned astronomers on Friday described the greatest cosmic explosion ever monitored - a star burst from the other side of the galaxy that was briefly brighter than the full moon and swamped satellites and telescopes.
"This is a once-in-a-lifetime event," said Rob Fender of Britain's Southampton University of the high-radiation flash detected on December 27.
"We have observed an object only 20km 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."
"It was the mother of all magnetic flares - a true monster," said Kevin Hurley, a research physicist at the University of California at Berkeley.
Bryan Gaensler of the United States' Harvard-Smithsonian Centre for Astrophysics, described the burst as "maybe a once per century or once per millennium event in our galaxy".
In our back yard
"Astronomically speaking, this explosion happened in our back yard. If it were in our living room, we'd be in big trouble."
The blast was caused by an eruption on the surface of a neutron star called SGR 1806-20, about 50 000 light years from Earth in the constellation of Sagittarius - about three billion times farther from us than the Sun.
A neutron star is the remnant of a very large star near the end of its life - a tiny, extraordinarily dense core with a powerful magnetic field, spinning swiftly on its axis.
When these ancient star cores finally run out of fuel, they collapse in on themselves and explode as a supernova.
There are millions of neutron stars in the Milky Way but, so far, only a dozen have been found to be "magnetars": neutron stars with an ultra-powerful magnetic field.
Magnetars have a magnetic field so powerful that it could strip the data off a credit card at a distance of 200 000km.
SGR 1806-20 is even rarer: one of only four known "soft gamma repeater" (SGR) magnetars, so called because they flare up randomly and release gamma rays in a mammoth burst.
Why this happens is unknown. One theory is that the energy release comes from magnetic fields which wrestle and overlap because of the star's spin and then snap back and reconnect, rather like the competing faults that cause an earthquake.
The SGR 1806-20 spewed out about 10 000 trillion trillion watts, or about 100 times brighter than any of the several "giant flares" that have been previously recorded.
Despite this energy loss, the strange star did not even pause, Britain's Royal Astronomical Society (RAS) said.
"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," the RAS said.
The flare, detected by satellites and telescopes operated by Nasa and Europe, was so powerful that it bounced off the moon and lit up the Earth's upper atmosphere. For over a tenth of the second, it was actually brighter than a full moon, and briefly overwhelmed delicate sensors, RAS said.
- AFP
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