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For about 20 years astrophysicists have suspected that the most rapidly spinning neutron stars known, the millisecond radio
pulsars, achieved their dizzying millisecond spin periods over a billion year accretion phase during which material is
transferred to the neutron star from a binary companion. This transfer of material, which occurs via an accretion disk orbiting
the neutron star, produces a torque on the neutron star that causes it slowly to spin faster and faster. Astronomers can study
this accretion process by observing the X-rays produced by material falling into the powerful gravitational field of the neutron
star; however, until recently no one had been able to see in the X-rays the pulsating signature of the rapidly spinning neutron
star which theorists predicted should be present. This remained one of the longest standing puzzles in X-ray astronomy. But now
with NASA's Rossi
X-ray Timing Explorer (RXTE) the first discovery of such an object has been made in April, 1998. The new pulsar, known as
SAXJ1808.4-3658, is spinning about 401 times per second and itself orbits its companion star once every two hours. The companion
star, with only about 0.1 of the mass of our Sun, may in fact be slowly vaporizing as the pulsar bombards it with powerful X-ray
and energetic particle beams.
This animation attempts to condense the billion year evolutionary history of such a binary system into a few tens of seconds.
It begins with two stars, one more massive than the other, in a tight orbit. The massive star evolves first and swallows up its
companion, which spirals into it forming an even tighter binary system. The core of the massive star produces a supernova and
leaves behind a neutron star. The neutron star's companion eventually begins to lose mass and forms an accretion disk around the
neutron star. The accretion of material onto the neutron star causes it to spin faster and faster, eventually reaching a spin
period of a few milliseconds. The accreted material produces X-rays which in turn can begin vaporizing the companion. All that
remains at the end is a highly compact, rapidly rotating neutron star which produces a pair of radio beams and may be observable
as a millisecond radio pulsar.
Read the full NASA Press
Further information is available from the RXTE Learning Center and Imagine the
Millisecond Pulsar Animation
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