  One of the most dramatic discoveries made so far with the Rossi X-Ray Timing
Explorer is that many accreting neutron stars with weak magnetic fields
generate strong, remarkably coherent, high-frequency X-ray brightness
oscillations. The 325-1200 Hz quasi-periodic oscillations (QPOs) observed in
the accretion-powered emission are almost certainly produced by gas orbiting
very close to the stellar surface and have frequencies related to the orbital
frequencies of the gas. The 360-600 Hz brightness oscillations seen during
thermonuclear X-ray bursts are produced by one or two hotter regions on the
stellar surface and have frequencies equal to the stellar spin frequency or its
first overtone. Measurements of these oscillations are providing tight upper
bounds on the masses and radii of neutron stars, and important new constraints
on the equation of state of neutron star matter.
