A group of scientists has announced that recent observations of one particularly tempestuous neutron star seems to bolster the theory that irregular gamma and x-ray bursts are caused by starquakes.
The team, led by Peter Woods, a research scientist at NASA's Marshall Space Flight Center in Huntsville, has been observing certain neutron stars known as soft gamma reflectors, or SGRs. Only four of these stars are known to exist. So named because they emit bursts of "soft," or low-level gamma rays at irregular intervals, scientists have been hard pressed to explain just what causes the bursts. Much easier to explain are the radiation flares of pulsars -- other neutron stars that flash at a constant rate as they rotate.
The SGR's, though, seem to flash with no particular pattern.
Last year, the gamma repeater known as SGR 1900+14 suddenly flashed to life after 20 years of relative dormancy.
"That's how these things behave. They're very quiet for long periods of time then all of a sudden they'll just go nuts," said Woods, who is also a graduate student in astrophysics at the University of Alabama at Huntsville.
Go nuts, it did.
The neutron star was identified as a source of gamma ray bursts in 1979, but it was only seen to flash seven times between then and 1998. Last May, though, the star flared up, and it has done so more than 200 times since. Each burst releases as much energy as the Sun does in a year.
"These flashes are really the starquakes," said Robert Duncan, a research astronomer at the University of Texas who has worked closely with Woods. "We believe that these events are triggered by breaking of the crust," he said.
Neutron stars are the only stars (along with some white dwarfs) that have a solid surface. They are thought to be the collapsed cores of stars, the remnants of immense explosions called supernovae. About 12 miles (20 kilometers) in diameter, neutron stars contain the mass of the Sun and have a superfluid interior covered by a metallic crust. That crust is thought to be slightly more than a half mile (about 1 kilometer) thick.
A starquake can be thought of as a displacement or rupture of the crust, similar to the tear between two of Earth's tectonic plates along a fault. A displacement as small as a few millimeters could create a typical gamma ray burst, Duncan said.
But the two-hundred-plus typical bursts were impotent flickers compared to the explosion of August 27. On that day the star erupted with a flash more than 1,000 times as powerful as any yet observed. The gamma ray flash lasted almost six minutes. It exploded as much energy during the first second as the sun releases in 1,000 years.
What most amazed Woods, though, was a dramatic deceleration in the star's spin rate -- what looked like the sudden breaking of the whirling magnetic star.
Scientists classify SGRs as magnetars, a 10-member family of neutron stars that has the peculiar characteristic of slowing in rotation very rapidly. The rate of this deceleration -- a few milliseconds per year -- is called the spindown rate. Theorists believe that a strong magnetic field is responsible for applying the breaks.
"During the summer of 1998, we observed a rapid change in the spindown, and it happened in a period when the burst source was very active," Woods said. After that 80-day period, it (the spindown) was about 2 times faster than it was previously."
In May 1998, the rate was measured to be about 2 milliseconds per year. It then increased to about 4.5 milliseconds per year, and today is back to 2 milliseconds per year, Woods said. A rotation now takes almost one two-hundredth of a second longer to complete than it did last year. This is an important difference in a star that spins once every 5.16 seconds.
Woods believes the August 27 burst caused the change.
His is a bold explanation because scientists have never observed a star decelerate at anything but a steady rate. Magnetars that are slowing down generally do so continuously, but in this case, Woods believes the magnetar's surface slowed down all at once. The action might be compared to that of a car on a highway exit ramp when the driver slams on the breaks for an instant during an otherwise steady deceleration.
This event may not have been a mere starquake, Duncan said. It was probably too powerful, and could be explained better using a model similar to a solar flare. That event probably required an immense upwelling of magnetic energy from the interior, which disrupted the surface and showered energy outward.
It could have caused the crust to lose angular momentum, transferring it either out to space or inward to the superfluid interior. In either case, the energy loss at the surface would be noticed as a dramatic spindown of the star's rotation, Woods said.
Although the physics of neutron stars and magnetars are not well understood, Woods said the observations made over the past year offer good evidence for the existence of starquakes.
"When you think about it, it's pretty amazing," Woods said. "SGRs are super dense objects with a high magnetic field and a very deep gravitational well. The Earth is just a little rock in comparison, but it looks like the same phenomenon is occurring."
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