The Origin of Kilohertz QPOs and Implications for Neutron Stars
  One of the most dramatic discoveries made with the Rossi X-Ray Timing Explorer is that many accreting neutron stars in low-mass binary systems produce strong, remarkably coherent, high-frequency X-ray brightness oscillations. The 325-1200 Hz quasi-periodic oscillations (QPOs) observed in the accretion-powered emission are thought to be produced by gas orbiting very close to the neutron star, whereas the 360-600 Hz brightness oscillations seen during thermonuclear X-ray bursts are produced by one or two hot spots rotating with the star and have frequencies equal to the stellar spin frequency or its first overtone. The oscillations constrain the masses and radii of these neutron stars, which are thought to be the progenitors of the millisecond pulsars. Modeling indicates that the stars have spin frequencies 250-350 Hz and magnetic fields 1e7-5e9 G. 
Frederick K. Lamb, M. Coleman Miller, and Dimitrios Psaltis