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Science
PARIS (AFP) - Stunned astronomers 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.
The high-radiation flash, detected last December 27, caused no harm to Earth but would have literally fried the planet had it occurred within a few light years of home.
Normally reserved skywatchers struggled for superlatives.
"This is a once-in-a-lifetime event," said Rob Fender of Britain's Southampton University.
"We have observed an object only 20 kilometers (12 miles) across, on the other side of our galaxy, releasing more energy in a 10th 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 Center for Astrophysics, described the burst as "maybe a once per century or once per millennium event in our galaxy.
"Astronomically speaking, this explosion happened in our backyard. 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 known, exotic kind of neutron star called SGR 1806-20, located about 50,000 light years from Earth in the constellation of Sagittarius and 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 have a magnetic field measuring about 1,000 trillion gauss, hundreds of times more powerful than that of any other object in the Universe.
To give an idea of this in earthly terms, the field is so powerful that it could strip the data off a credit card at a distance of 200,000 kilometers (120,000 miles).
SGR 1806-20 is an even rarer bird. It is 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, creating a "starquake" rather like the competing faults that cause an earthquake.
What is sure, though, is that the outpouring of energy is massive.
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 (news - web sites) 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.
Two science teams, formed by observations provided by 20 institutes around the world, will report on the blast in a forthcoming issue of the British weekly journal Nature.
Many questions will be thrown up by the event, including the intriguing speculation that the dinosaurs may have been wiped out by a similar, closer gamma-ray explosion 65 million years ago, and not by climate change inflicted by an asteroid impact.
"Had this happened within 10 light years of us, it would have severly damaged our atmosphere and possibly have triggered a mass extinction," said lead-author Gaensler.
The good news, he noted, is that the nearest known magnetar to Earth, 1E 2259+586, is about 13,000 light years away.
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FOLLOWUP By STEPHEN MANNING, Associated Press Writer
GREENBELT, Md. - A massive burst of energy exploded from a far-off neutron star last December, the brightest flash of light ever detected from beyond the solar system, scientists said Friday.
The Dec. 27 flare was by far the largest of three such giant outbursts of gamma rays detected in the last 35 years from neutron stars, the densely packed and supercharged remnants of a collapsed star.
"This is a once-in-a-lifetime event," David Palmer, a scientist at the Los Alamos National Laboratory in New Mexico and lead author of a paper on the flare.
The energy burst, packing more energy than the sun emits every 150,000 years, was not visible to humans, and the gamma rays were blocked by the Earth's atmosphere as they rushed by. Scientists said some operators of low-frequency transmitters were able to detect it.
NASA (news - web sites)'s new observatory — named Swift for its speedy pivoting and pointing — is among the instruments that detected the flare. It was launched last November to probe the workings of black holes. The satellite, controlled by scientists at the Goddard Space Flight Center in Greenbelt, is designed to detect gamma ray outbursts and quickly pivot to record them. It also recorded the afterglow of the blast.
"Swift, within a mere month of its launch, was able to participate in an amazing discovery," said Roger Blandford, a physicist at Stanford University.
Neutron stars are formed when massive stars run out of fuel and collapse, creating dense, fast-spinning and highly magnetic solar corpses that are only about 15 miles in diameter.
The December burst lasted a tenth of second and came from a neutron star about 50,000 light years away from Earth in the constellation Sagittarius. Called SGR 1806-20, it is one of only about 12 known magnetars, a neutron star with a magnetic field that is trillions of times stronger than that of Earth.
Scientists believe the magnetic field of the magnetars can shift like an earthquake, causing it to eject a huge burst of energy. SGR 1806-20 is know as a "soft gamma repeater" because the initial flare is followed by a series of much smaller releases of gamma rays. The December flare was up to a billion times more powerful than typical flares from soft gamma repeaters.
The aftermath of the blast is a smoldering oblong ring that glows for several days after the flare caused by debris launched into the gas surrounding the star.
Ref.
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