Massive quake leaves neutron star ringing note of “F sharp”
July 13, 2005
Courtesy University of California at San Diego
and World Science staff
A massive quake in a neutron star has left the whole super-dense object ringing like a bell, researchers say—and if we could hear it, it would be a slightly out-of-tune version of a piano’s 22nd key, an F sharp.
NASA released an animation of the
burst, which researchers said occurred halfway across our Milky Way galaxy and was the largest explosion of
its type ever recorded.
The event should allow researchers for the first time to probe what’s inside neutron stars, super-compact objects believed to have a consistency like that of a gigantic atomic nucleus.
An international team of astrophysicists, combing through data from a NASA X-ray satellite, the Rossi X-ray Timing Explorer, described the explosion in the July 20th issue of
Astrophysical Journal Letters, a research journal.
They said it 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 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’s Center for Astrophysics and Space Sciences 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 solar 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 inside 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 international team found the oscillations from data it retrieved two days after Christmas by the Rossi X-Ray Timing Explorer, a satellite designed to study the fluctuating X-ray emissions from stellar sources.
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 sphere of neutrons is so dense—packing the mass of the sun in a sphere only 10 miles in diameter—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 and are 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 the 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 from this unusually large explosion could have come from a highly twisted magnetic field surrounding the neutron star that suddenly snapped, creating a titanic quake on the neutron 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. “With this energy release, the magnetic field surrounding the magnetar was presumably able to relax to a more stable configuration.”
“The sudden and surprising occurrence of this giant flare, which will help us learn more about the nature of magnetars and the internal make-up of neutron stars,” said Rothschild, “underlines the importance of having satellites and telescopes with the capacity to record unusual and unpredictable phenomena in the universe.”