Using observations
from NASA's Rossi X-ray Timing Explorer (RXTE), an international team of
astronomers has discovered a timing mechanism that allows them to predict
exactly when a superdense star will unleash incredibly powerful explosions.
"We found a clock that ticks slower and slower, and when it slows down
too much, boom! The bomb explodes," says lead author Diego Altamirano of
the University of Amsterdam in the Netherlands.
The bursts occur on a neutron star, which is the collapsed remnant of a
massive star that exploded in a supernova. The neutron star belongs to a
binary system that can be described as a ticking time bomb. Hydrogen and
helium gas from a companion star spirals onto the neutron star, slowly
accumulating on its surface until it heats up to a critical temperature.
Suddenly, the hydrogen and helium begin to fuse uncontrollably into heavier
elements, igniting a thermonuclear flame that quickly spreads around the
entire star. The resulting explosion appears as a bright flash of X-rays.
These bursts, which can occur several times per day from the same
neutron star, release more energy in just 10 to 100 seconds than our Sun
radiates in an entire week. Put another way, the energy is equivalent to
100 fifteen-megaton hydrogen bombs exploding simultaneously over each
postage-stamp-size patch of the neutron star's surface.
Scientists have observed thousands of these X-ray bursts from about 80
different neutron stars. But until now, they had no way to predict when
they would occur.
The key to this discovery is RXTE, which makes extremely precise timing
measurements of rapidly flickering X-ray-emitting objects. As gas gradually
builds up on the neutron star's surface, hydrogen and helium atoms
sometimes fuse into heavier elements in a stable and almost perfectly
repetitive fashion. This mode of fusion produces a nearly regular X-ray
signal known as a quasi-periodic oscillation, or QPO for short. Theory
predicts that the frequency of the cycle should be about 0.009 cycles per
second (9 Millihertz, or one cycle every two minutes). This is very close
to the QPO frequency in 4U 1636-53 measured by Altamirano and his
colleagues using extensive RXTE observations.
But the team also found that the QPO frequency decreased over time from
about 12 Millihertz to 8 Millihertz. In a paper published recently in
Astrophysical Journal Letters, the authors demonstrate that every time the
QPO frequency slowed down to about 8 Millihertz (one cycle per 125
seconds), the neutron star in 4U 1636-53 let loose a powerful X-ray burst.
"We are able to predict when these explosions are happening. We have a
clock that tells us when the bomb will explode!" says Altamirano.
"We do not yet know if this sequence of events means the oscillations
cause the explosion, or if they are just telling us the time has come for
an outburst. Further observations from RXTE will be essential to figure
this out," adds coauthor Michiel van der Klis, who also works at the
University of Amsterdam.
The same group is now studying more than 50 other neutron stars to see
if it can identify similar behavior. The 4U 1636-53 system is located about
20,000 light-years away right near the border between the southern
constellations of Ara and Norma.
"It's an exciting discovery," says astrophysicist Tod Strohmayer of
NASA's Goddard Space Flight Center in Greenbelt, Md. "The QPO frequencies
are related to the mass and size of the neutron star, so we may be able to
use them to pin down the masses of some neutron stars. It gives us a new
tool to study these fascinating objects."
SOURCE NASA
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