  Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered
that disk-accreting neutron stars with weak magnetic fields produce three
distinct types of high-frequency X-ray oscillations. These oscillations are
powered by release of the binding energy of matter falling into the strong
gravitational field of the star or by the sudden nuclear burning of matter that
has accumulated in the outermost layers of the star. The frequencies of the
oscillations reflect the orbital frequencies of gas deep in the gravitational
field of the star and/or the spin frequency of the star. These oscillations can
therefore be used to explore fundamental physics, such as strong-field gravity
and the properties of matter under extreme conditions, and important
astrophysical questions, such as the formation and evolution of millisecond
pulsars. Observations using RossiXTE have shown that some two dozen neutron
stars in low-mass X-ray binary systems have the spin rates and magnetic fields
required to become millisecond radio-emitting pulsars when accretion ceases,
but that few have spin rates above about 600 Hz. The properties of these stars
show that the paucity of spin rates greater than 600 Hz is due in part to the
magnetic braking component of the accretion torque and to the limited amount of
angular momentum that can be accreted in such systems. Further study will show
whether braking by gravitational radiation is also a factor. Analysis of the
kilohertz oscillations has provided the first evidence for the existence of the
innermost stable circular orbit around dense relativistic stars that is
predicted by strong-field general relativity. It has also greatly narrowed the
possible descriptions of ultradense matter.
