WORLD SCIENCE

"Long before it's in the papers"
December 20, 2005

July 13, 2005
Courtesy University of California at San Diego
and World Science staff

A massive quake in a neutron star left the super-dense star ringing like a bell, researchers say—and if we could have heard it, it would have sounded a slightly mistuned version of the note from a piano’s 22nd key, called F sharp.

An artist's conception of the way gamma radiation expanded away from a neutron star in a giant bubble at light speed. Later, the surface of that bubble reached Earth to be picked up by detectors here. (NASA) An animation shows the expanding bubble of gamma rays as it first leaves the star, then, tens of thousands of years later, reaches Earth.

Scientists say this vibration will let them for the first time probe what’s inside a neutron star, a super-compact object believed to be something like a giant atomic nucleus.

NASA also released an animation of an explosion that accompanied the quake. Researchers said it occurred halfway across our Milky Way galaxy and was the largest of its type ever recorded.

An international team of astrophysicists described the blast in the July 20 issue of Astrophysical Journal Letters, a research journal.

They said the burst produced vibrations in the star that generated quick fluctuations in the X-ray radiation it released into space. These pulses are emitted during each seven-second rotation of the fast-spinning star and reveal the frequency, or speed, of the star’s vibrations.

“This explosion was akin to hitting the neutron star with a gigantic hammer, causing it to ring like a bell,” said Richard Rothschild, an astrophysicist at the University of California and one of the authors of the report. 

“Now the question is: what does the frequency of the neutron star’s oscillations—the tone produced by the ringing bell—mean?”

Much as geologists probe the Earth’s interior from seismic waves produced by earthquakes, and astronomers study the sun using shock waves traveling through the sun, astrophysicists say they can learn about what’s inside neutron stars from these fluctuations.

“Does it mean neutron stars are just a bunch of neutrons packed together?” Rothschild asked. “Or do neutron stars have exotic particles, like quarks, at their centers as many scientists believe?” 

According to the traditional model of a neutron star—a shrunken, compacted remnant of a dead normal star—it contains only neutrons, the electrically neutral particles that also inhabit atomic nuclei. But some more complex theories of neutron stars propose that in their cores, the extreme pressures melt neutrons into still more elementary particles, known as quarks.

Another question scientists want to answer is “how does the crust of a neutron star float on top of its superfluid core?” Rothschild said. A superfluid is a type of liquid that flows with no friction.

“This is a rare opportunity for astrophysicists to study the interior of a neutron star,” Rothschild said. 

The quakes ripped through the star at a stupefying speed, vibrating the star at 94.5 cycles per second, researchers said. “This is near the frequency of the 22nd key of a piano, F sharp,” said Tomaso Belloni of Italy’s National Institute of Astrophysics, a member of the team who measured the signals. 

The team found the oscillations from data retrieved two days after Christmas from the Rossi X-Ray Timing Explorer, a NASA satellite designed to study X-ray emissions from stars. 

The peculiar oscillations the researchers found began three minutes after a titanic explosion on the star that, for only a tenth of a second, released more energy than the sun emits in 150,000 years, they said. 

The oscillations then gradually receded after about 10 minutes.

Neutron stars are the dense, rapidly spinning cores of material that result from the crushing collapse of a star that has consumed its fuel and exploded. 

The collapse is so intense that electrons are forced into the atomic nucleus and combine with protons to become neutrons. The resulting object is so dense—packing the weight of the sun into a ball only 10 miles (16 kilometers) wide—that a spoonful of its matter would weigh billions of tons on Earth.

Most of the millions of neutron stars in our Milky Way galaxy produce magnetic fields a trillion times stronger than those of the Earth. But astrophysicists have discovered less than a dozen ultra-high magnetic neutron stars, called “magnetars,” with magnetic fields a thousand times greater—strong enough to strip information from a credit card at a distance halfway to the moon.

These intense magnetic fields are strong enough they sometimes buckle the crust of neutron stars, causing “star quakes” that result in the release of gamma rays, a more energetic form of radiation than X-rays. 

Four of these magnetars are known to do just that. They’re called “soft gamma repeaters” because they flare up randomly and release a series of brief bursts of gamma rays. The neutron star that researchers are studying is one of these.

SGR 1806-20, as it is called, sent X-rays flooding through the galaxy on December 27—producing a flash brighter than anything ever detected beyond our solar system. The flash was so bright that it blinded all X-ray satellites in space for an instant and lit up the Earth’s upper atmosphere.

Astrophysicists suspect the burst of gamma-ray and X-ray radiation could have come from a highly twisted magnetic field surrounding the neutron star that suddenly snapped, creating a titanic quake on the star.

“The scenario was probably analogous to a twisted rubber band that finally broke and in the process released a tremendous amount of energy,” said Rothschild. The flare’s surprise appearance, he added, “underlines the importance of having satellites and telescopes with the capacity to record unusual and unpredictable phenomena in the universe.”

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