NASA satellite pins down timer in stellar ticking time bomb (Forwarded)

Robert Naeye / Rob Gutro
Goddard Space Flight Center, Greenbelt, Md. April 30, 2008
301-286-4453/4044

PRESS RELEASE: 08-35

NASA SATELLITE PINS DOWN TIMER IN STELLAR TICKING TIME BOMB

GREENBELT, Md. -- 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."

For related images to this story, please visit on the Web:

http://www.nasa.gov/centers/goddard/...mechanism.html