The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U   1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
  We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We find that between two of these tracks, for similar QPO frequency, the source count rate can differ by up to a factor of ~ 4, whereas at the same time the rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not depend upon which track the source occupies in the frequency vs. X-ray count rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with scenarios in which the properties of the kHz QPOs are only determined by the mass accretion rate through the disk, whereas X-ray count rate also depends upon other sources of energy that do not affect the QPOs. 
Mariano Mendez, Michiel van der Klis, and Eric C. Ford (University of   Amsterdam)