Stunned astronomers on Friday described the greatest cosmic explosion monitored to date — 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 20km 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’s Harvard-Smithsonian Centre 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.”
Neutron stars
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 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 000km.
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 that 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.
Star ‘did not even pause’
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 more than a 10th 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 severely 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. — Sapa-AFP