The amplitude of the kilohertz quasi-periodic oscillations in 4U 1728-34, 4U 1608-52, and Aquila X-1, as a function of X-ray intensity

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-ma ss X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces o ut a set of nearly parallel tracks in a frequency versus X-ray count rate d iagram. We find that between two of these tracks, for similar QPO frequency , the 2-60 keV source count rate can differ by up to a factor of similar to 4, whereas at the same time the rms amplitude of the kHz QPOs is only a fac tor of similar to1.1 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 s ource occupies in the frequency versus X-ray count rate diagram. Our result s for 4U 1728-34, 4U 1608-52, and Aql X-1 are inconsistent with simple "ext ra source of X-rays" scenarios for the parallel tracks, such as those in wh ich the properties of the kHz QPOs are determined only by the mass accretio n rate through the disk, whereas X-ray count rate also depends upon other s ources of energy that do not affect the QPOs.