X-Ray Oscillations From Biggest Star Quake In Universe
Provide Clues to Mysterious Interior of Neutron Stars
issued by University of California
July 12, 2005 San Diego -- A gigantic explosion on a neutron star halfway across the Milky Way
galaxy, the largest such explosion ever recorded in the universe, should
allow astronomers for the first time to probe the interiors of these
mysterious stellar objects.
An international team of astrophysicists, combing through data from a NASA
X-ray satellite, the Rossi X-ray Timing Explorer, reports in the July 20th
issue of Astrophysical Journal Letters that the explosion produced
vibrations within the star, like a ringing bell, that generated rapid
fluctuations in the X-ray radiation it emitted into space. These X-ray
pulses, emitted during each seven second rotation by the fast-spinning
star, contained the frequency vibrations of the neutron star's massive
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, the X-ray fluctuations discovered from this
explosion should provide critical information about the internal structure
of neutron stars.
"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 journal report. Now the
question is, What does the frequency of the neutron star's
oscillations the tone produced by the 'ringing bell' mean?
"Does it mean neutron stars are just a bunch of neutrons packed together?
Or do neutron stars have exotic particles, like quarks, at their centers
as many scientists believe? And how does the crust of a neutron star float
on top of its superfluid core? This is a rare opportunity for
astrophysicists to study the interior of a neutron star, because we
finally have some data theoreticians can chew on. Hopefully, they‚ll be
able to tell us what this all means."
The quakes ripped through the neutron star at an incredible speed,
vibrating the star at 94.5 cycles per second. "This is near the frequency
of the 22nd key of a piano, F sharp," said Tomaso Belloni, an Italian
member of the team who measured the signals.
The international team-led by GianLuca Israel, Luigi Stella and Belloni of
Italy's National Institute of Astrophysics-discovered 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 a neutron star 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
Neutron stars are the dense, rapidly spinning cores of matter that result
from the crushing collapse of a star that has depleted all of its nuclear
fuel and exploded in a cataclysmic event known as a supernova. The
collapse is so crushing 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
Most of the millions of neutron stars in our Milky Way galaxy produce
magnetic fields that are 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 termed "soft gamma
repeaters", or SGRS, by astrophysicists because they flare up randomly and
release a series of brief bursts of gamma rays.
SGR 1806-20, the formal designation of the neutron star that exploded and
sent X-rays flooding through the galaxy on December 27, 2004-producing a
flash brighter than anything ever detected beyond the solar systemųis one
of them. 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 December 27 flash of energy was detected by several other NASA and
European satellites and recorded by radio telescopes around the world. It
already has been the subject of numerous scientific papers published in
"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."
Other members of the international team were Pier Giorgio Casella, Simone
Dall‚Osso and Massimo Persic of Italy‚s National Institute of
Astrophysics; Yoel Rephaeli of UCSD 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.