X-Ray Oscillations From Biggest Star Quake In Universe
Provide Clues To Mysterious Interior of Neutron Stars
By Kim McDonald
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
conception of the December 27, 2004 gamma ray flare expanding from SGR 1806-20 and impacting Earth’s
here to view animation (no audio). Credit: NASA
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 quakes.
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.
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?
of thousands of years ago, a “starquake”
fractured the magnetar’s surface. The result was an
explosive release of energy, which sent a pulse of gamma
rays racing across the cosmos at the
speed of light. Click
here to view animation (no audio). Credit: NSF
“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 minutes.
Rothschild of UCSD with instrument that
measured X-rays from exploding neutron star.
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 Earth.
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
system—is 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.
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.
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 recent months.
“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
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.
Media Contact: Kim McDonald
Rothschild (858) 534-3462