Huge 'Star-Quake' Rocks Milky Way
BBC News - 20 February 2005
Astronomers say they have been stunned by the amount of energy released
in a star explosion on the far side of our galaxy, 50,000 light-years
away.
The flash of radiation on 27 December was so powerful that it bounced
off the Moon and lit up the Earth's atmosphere.
The blast occurred on the surface of an exotic kind of star - a
super-magnetic neutron star called SGR 1806-20. If the explosion
had been within just 10 light-years, Earth could have suffered a
mass extinction, it is said.
"We figure that it's probably the biggest explosion observed
by humans within our galaxy since Johannes Kepler saw his supernova
in 1604," Dr Rob Fender, of Southampton University, UK, told
the BBC News website.
One calculation has the giant flare on SGR 1806-20 unleashing about
10,000 trillion trillion trillion watts. "This is a once-in-a-lifetime
event. 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," said Dr Fender.
Fast turn
The event overwhelmed detectors on space-borne telescopes, such
as the recently launched Swift observatory.
This facility was put above the Earth to detect and analyze gamma-ray
bursts - very intense but fleeting flashes of radiation. The giant
flare it and other instruments caught in December has left scientists
scrabbling for superlatives.
Twenty institutes from around the world have joined the investigation
and two teams are to report their findings in a forthcoming issue
of the journal Nature.
The light detected from the giant flare was far brighter in gamma-rays
than visible light or X-rays.
Research teams say the event can be traced to the magnetar SGR 1806-20.
This remarkable super-dense object is a neutron star - it is composed
entirely of neutrons and is the remnant collapsed core of a once
giant star. Now, though, this remnant is just 20km across and spins
so fast it completes one revolution every 7.5 seconds.
"It has this super-strong magnetic field and this produces
some kind of structure which has undergone a rearrangement - it's
an event that is sometimes characterised as a 'star-quake', a neutron
star equivalent of an earthquake," explained Dr Fender. "It's
the only possible way we can think of releasing so much energy."
Continued glow
SGR 1806-20 is sited in the southern constellation Sagittarius.
Its distance puts it beyond the centre of the Milky Way and a safe
distance from Earth.
"Had this happened within 10 light-years of us, it would have
severely damaged our atmosphere and would possibly have triggered
a mass extinction," said Dr Bryan Gaensler, of the Harvard-Smithsonian
Center for Astrophysics, who is the lead author on one of the forthcoming
Nature papers. "Fortunately there are no magnetars anywhere
near us."
The initial burst of high-energy radiation subsided quickly but
there continues to be an afterglow at longer radio wavelengths.
This radio emission persists as the shockwave from the explosion
moves out through space, ploughing through nearby gas and exciting
matter to extraordinary energies. "We may go on observing this
radio source for much of this year," Dr Fender said.
This work is being done at several centres around the globe, including
at the UK's Multi-Element Radio-Linked Interferometer Network (Merlin)
and the Joint Institute for VLBI (Very Long Baseline for Interferometry)
in Europe - both large networks of linked radio telescopes.
© BBC MMV
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