GREENBELT, Md., Jan. 2 /PRNewswire-USNewswire/ -- New observations from
Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and NASA X-ray
observatory, has challenged scientists' conventional understanding of white
dwarfs. Observers had believed white dwarfs were inert stellar corpses that
slowly cool and fade away, but the new data tell a completely different
story.
At least one white dwarf, known as AE Aquarii, emits pulses of
high-energy (hard) X-rays as it whirls around on its axis. "We're seeing
behavior like the pulsar in the Crab Nebula, but we're seeing it in a white
dwarf," says Koji Mukai of NASA Goddard Space Flight Center in Greenbelt,
Md. The Crab Nebula is the shattered remnant of a massive star that ended
its life in a supernova explosion. "This is the first time such pulsar-like
behavior has ever been observed in a white dwarf." Mukai is co-author of a
paper presented at a Suzaku science conference in San Diego, Calif., in
December.
White dwarfs and pulsars represent distinct classes of compact objects
that are born in the wake of stellar death. A white dwarf forms when a star
similar in mass to our sun runs out of nuclear fuel. As the outer layers
puff off into space, the core gravitationally contracts into a sphere about
the size of Earth, but with roughly the mass of our sun. The white dwarf
starts off scorching hot from the star's residual heat. But with nothing to
sustain nuclear reactions, it slowly cools over billions of years,
eventually fading to near invisibility as a black dwarf.
A pulsar is a type of neutron star, a collapsed core of an extremely
massive star that exploded in a supernova. Whereas white dwarfs have
incredibly high densities by earthly standards, neutron stars are even
denser, cramming roughly 1.3 solar masses into a city-sized sphere. Pulsars
give off radio and X-ray pulsations in lighthouse-like beams.
The discovery team, led by Yukikatsu Terada of the Institute of
Physical and Chemical Research (RIKEN) in Wako, Japan, was not expecting to
find a white dwarf mimicking a pulsar. Instead, the astronomers were hoping
to find out if white dwarfs could accelerate charged subatomic particles to
near-light speed, meaning they could be responsible for many of the cosmic
rays that zip through our galaxy and occasionally strike Earth.
Some white dwarfs, including AE Aquarii, spin very rapidly and have
magnetic fields millions of times stronger than Earth's. These
characteristics give them the energy to generate cosmic rays.
To find out if this is happening, Terada and his colleagues targeted AE
Aquarii with Suzaku in October 2005 and October 2006. The white dwarf
resides in a binary system with a normal companion star. Gas from the star
spirals toward the white dwarf and heats up, giving off a glow of
low-energy (soft) X-rays. But Suzaku also detected sharp pulses of hard
X-rays. After analyzing the data, the team realized that the hard X-ray
pulses match the white dwarf's spin period of once every 33 seconds.
The hard X-ray pulsations are very similar to those of the pulsar in
the center of the Crab Nebula. In both objects, the pulses appear to be
radiated like a lighthouse beam, and a rotating magnetic field is thought
to be controlling the beam. Astronomers think that the extremely powerful
magnetic fields are trapping charged particles and then flinging them
outward at near-light speed. When the particles interact with the magnetic
field, they radiate X-rays.
"AE Aquarii seems to be a white dwarf equivalent of a pulsar," says
Terada. "Since pulsars are known to be sources of cosmic rays, this means
that white dwarfs should be quiet but numerous particle accelerators,
contributing many of the low-energy cosmic rays in our galaxy."
Launched in 2005, Suzaku is the fifth in a series of Japanese
satellites devoted to studying celestial X-ray sources. Managed by JAXA,
this mission is a collaborative effort between Japanese universities and
institutions and Goddard.
For related images to this story, please visit on the Web:
http://www.nasa.gov/centers/goddard/news/topstory/2007/whitedwarf_pulsa
r.html
For more information on Suzaku, please visit:
http://suzaku.gsfc.nasa.gov
http://www.isas.jaxa.jp/e/
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