The surface patterns for different torsional modes that
may have been excited by the hyperflare. The colors and arrow
lengths indicate the magnitude of the vibrations.
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.
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 that 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 harbor 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
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 12 miles across.
The two scientists examined a neutron star named SGR 1806-20, about
40,000 light years from Earth in the constellation Sagittarius. The
object is in a subclass of highly magnetic neutron stars called
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). 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
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 traveling around the star's crust and from those traveling
radially through it. The diameter of a neutron star is uncertain, but
based on the estimate of about 12 miles across, the crust would be
about 1 mile 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
"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."
T. E. Strohmayer und A. L. Watts
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
The Astrophysical Journal, 637, L117, (2006)
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, (2005)
Dr. Tod Strohmayer
NASA Goddard Space Flight Center, Greenbelt, MD, USA
Dr. Anna Watts
Max Planck Institute for Astrophysics, Garching, Germany