NASA Satellite Pins
Down Timer In Stellar Ticking Time Bomb
(30 April 2008) 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.
These four images from a computer animation illustrate a thermonuclear explosion as it ignites, spreads, and engulfs an entire neutron star. (courtesy: NASA)
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 co-author 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 behaviour.
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 Goddard Space Flight Center)
