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A US-German team of scientists from the Max Planck Institute
for Astrophysics and NASA have used NASA's Rossi X-ray Timing
Explorer to estimate the depth of the crust on a neutron star, the
densest object known in the universe. The crust, they say, is
approximately 1.6 kilometres deep and so tightly packed that a
teaspoon of this material would weigh about 10 million tonnes on
Earth.
Fig.1: The surface patterns for different torsional modes that
may have been excited by the hyperflare. The colours and arrow
lengths indicate the magnitude of the vibrations.
Image: Max Planck Institute for Astrophysics
This measurement, the first of its kind, came courtesy of a
massive explosion on a neutron star in December 2004. Vibrations
from the explosion revealed details about the star's composition.
The technique is analogous to seismology, the study of seismic
waves from earthquakes and explosions, which reveal the structure
of the Earth's crust and interior.
This new seismology
technique provides a way to probe a neutron star's interior, a
place of great mystery and speculation. Pressure and density are
so intense here that the core might harbour exotic particles
thought to have existed only at the moment of the Big Bang.
Dr
Anna Watts, of the Max Planck Institute for Astrophysics in
Garching, carried out this research in collaboration with Dr. Tod
Strohmayer of NASA's Goddard Space Flight Center in Greenbelt,
Maryland.
"We think this explosion, the biggest of
its kind ever observed, really jolted the star and literally
started it ringing like a bell," said Strohmayer. "The
vibrations created in the explosion, although faint, provide very
specific clues about what these bizarre objects are made of. Just
like a bell, a neutron star's ring depends on how waves pass
through layers of differing density, either slushy or solid."
A neutron star is the core remains of a star once several
times more massive than the sun. A neutron star contains about 1.4
solar masses of material crammed into a sphere only about 20
kilometres across. The two scientists examined a neutron star
named SGR 1806-20, which is situated about 40,000 light years from
Earth in the constellation Sagittarius. The object is in a
subclass of highly magnetic neutron stars called magnetars.
Fig. 2: The X-ray countrate for the giant flare, showing the
main flare at time zero and the decaying tail. The regular pulses
that can be seen are caused by a fireball of hot plasma that is
trapped close to the stellar surface. It swings in and out of our
field of view as the neutron star rotates. The more rapid seismic
oscillations are too fast to be visible on this plot, but start to
appear about 50 seconds after the main flare.
Image: Max Planck Institute for Astrophysics
On December 27, 2004, the surface of SGR 1806-20 experienced an
unprecedented explosion, the brightest event ever seen from beyond
our solar system. The explosion, called a hyperflare, was caused
by a sudden change in the star's powerful magnetic field that
cracked the crust, likely producing a massive starquake. The event
was detected by many space observatories, including the Rossi
Explorer, which observed the X-ray light emitted.
Strohmayer
and Watts think that the oscillations are evidence of global
torsional vibrations within the star's crust. These vibrations are
analogous to the S-waves observed during terrestrial earthquakes,
like a wave moving through a rope (see Figure 2). Their study,
building on observations of vibrations from this source by Dr.
GianLuca Israel of Italy's National Institute of Astrophysics,
found several new frequencies during the hyperflare.
Watts
and Strohmayer subsequently confirmed their measurements using
NASA's Ramaty High Energy Solar Spectroscopic Imager, a solar
observatory that also recorded the hyperflare, and found the first
evidence for a high-frequency oscillation at 625 Hz, indicative of
waves traversing the crust vertically.
The abundance of
frequencies - similar to a chord, as opposed to a single note -
enabled the scientists to estimate the depth of the neutron star
crust. This is based on a comparison of frequencies from waves
travelling around the star's crust and from those travelling
radially through it. The diameter of a neutron star is uncertain,
but based on the estimate of about 20 kilometres across, the crust
would be about 1.6 kilometres deep. This figure, based on the
observed frequencies, is in line with theoretical estimates.
Starquake seismology holds great promise for determining
many neutron star properties. Strohmayer and Watts have analyzed
archived Rossi data from a dimmer 1998 magnetar hyperflare (from
SGR 1900+14) and found telltale oscillations here too, although
not strong enough to determine the crust thickness.
A
larger neutron star explosion detected in X-rays might reveal
deeper secrets, such as the nature of matter at the star's core.
One exciting possibility is that the core might contain free
quarks. Quarks are the building blocks of protons and neutrons,
and under normal conditions are always tightly bound together.
Finding evidence for free quarks would aid in understanding the
true nature of matter and energy. Laboratories on Earth, including
massive particle accelerators, cannot generate the energies needed
to reveal free quarks.
"Neutron stars are great
laboratories for the study of extreme physics," said Watts.
"We'd love to be able to crack one open, but since that's
probably not going to happen, observing the effects of a magnetar
hyperflare on a neutron star is perhaps the next best thing."
Original work:
T.E.Strohmayer & A.L.Watts
Discovery of fast X-ray oscillations during the 1998 giant
flare from SGR 1900+14
The Astrophysical Journal, 632,
L111
A.L.Watts & T.E.Strohmayer
Detection with
RHESSI of high frequency X-ray oscillations in the tail of the
2004 hyperflare from SGR 1806-20
Astrophysical Journal, 637,
L117
- courtesy of Max Planck Institute for Astrophysics
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