The Harmonic and Sideband Structure of the Kilohertz Quasi-Periodic   Oscillations in Sco X-1
  We use data from the Rossi X-ray Timing Explorer to search for harmonics and sidebands of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in Sco X-1. We do not detect any of these harmonics or sidebands, with 95% confidence upper limits to their power between ~ 1% and ~ 10% of the power of the upper kHz QPO. The oscillations produced at these frequencies may be attenuated in a scattering corona around the neutron star. We find that upper limits to the unattenuated power of some of the strongest theoretically predicted harmonics and sidebands are as low as ~ 2% of the unattenuated power of the high-frequency QPO in Sco X-1. 
Mariano Mendez and Michiel van der Klis (University of Amsterdam)