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Starquake May Reveal Inside of a Neutron Star RXTE

A detailed look at data from an unprecedented neutron star explosion last year may reveal what's inside a neutron star, a major mystery in astronomy, scientists say.

The December 27, 2004, star explosion was the brightest ever recorded, detected by scores of telescopes; and it was the subject of even more scientific and popular-level articles. What's new, according to an international team of astrophysicists, is the detection of vibrations from the star, painstakingly extracted from data from NASA's Rossi X-ray Timing Explorer.

An artist conception of the SGR 1806 20 magnetar.

Image/animation above: Image: An artist conception of the SGR 1806-20 magnetar including magnetic field lines. After the initial flash, smaller pulsations in the data suggest hot spots on the rotating magnetar's surface. The data also shows no change in the magentar's rotation after the initial flash. Click on image to view animation (no audio). Credit: NASA

In the same way that seismic waves produced by earthquakes reveal features of the Earth's interior, these "starquake" vibrations might at long last provide information about the interior of neutron stars. Details of the findings are reported in The Astrophysical Journal Letters.

"This explosion was akin to hitting the neutron star with a gigantic hammer, causing it to ring like a bell," said Dr. Richard Rothschild, an astrophysicist at the University of California's Center for Astrophysics and Space Sciences in San Diego, one of the authors of the journal report. "Now the question is, what does the frequency of the neutron star's oscillations -- the tone produced by the ringing bell -- mean?"

Rothschild said that further analysis and calculations will be needed to determine the precise nature of the neutron star interior and that this observation is a solid steppingstone in that direction.

A neutron star is the core remains of a massive star that ran out of fuel and exploded, an event called a supernova. The core is incredibly dense, packing about a sun's worth of mass into a sphere about 15 miles across. Neutron stars are made of, well, neutrons. The material is so dense that protons and electrons are squeezed together to create neutrons. Yet the exact configuration is an utter mystery.

It could be that a solid crust of metals floats upon a superfluid "sea" of neutrons. Some scientists say that the neutron star core is filled with exotic, sub-atomic particles.  Earthbound laboratories cannot come close to reproducing the density of a neutron star. So neutron stars, hundreds to thousands of light years from Earth, serve as laboratories to observe extreme physics.

Scientists at Italy's National Institute of Astrophysics -- led by Drs. GianLuca Israel, Luigi Stella and Tomaso Belloni -- discovered what they describe as vibrations from a source called SGR 1806-20. Of the millions of neutron stars in the Milky Way galaxy, SGR 1806-20 is one of only four known neutron stars with a unique property of occasionally releasing great flares of gamma-ray light. Scientists call this kind of neutron star a soft gamma-ray repeater.

SGR 1806-20 has a very strong magnetic field billions of times greater than the Earth or Sun. The twisting and unraveling of these magnetic fields is thought to be the engine driving the gamma-ray flashes, like a rubber band that snaps. (SGR 1806-20 is also one of about a dozen known "magnetars," ultra-magnetic neutron stars.)

Combing through the Rossi X-ray data from the December 27 burst, the Italian team found flickering of the X-ray signal at a frequency of 94.5 cycles per second. "This is near the frequency of the 22nd key of a piano, F sharp," said Belloni, who measured the signals."

One explanation of this rapid flickering is that the neutron star surface itself is moving up and down nearly 100 times a second. The oscillations began three minutes after a titanic explosion on SGR 1806-20 that, for only a tenth of a second, released more energy than the Sun emits in 150,000 years. The oscillations then gradually receded after about 10 minutes.

"This is very precise data, the first of its kind, which theoreticians can now chew on," said Rothschild. "Hopefully, they'll be able to tell us what this all means."

Other members of the international team are Drs. Pier Giorgio Casella, Simone Dall'Osso and Massimo Persic of Italy's National Institute of Astrophysics; Yoel Rephaeli of University of California, San Diego, and the University of Tel Aviv; Duane Gruber, formerly of UCSD and now at the Eureka Scientific Corporation in Oakland, Calif; and Nanda Rea of the National Institute for Space Research in the Netherlands.

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