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-massX-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out aset of nearly parallel lines in a frequency vs. X-ray count rate diagram. Wefind that between two of these tracks, for similar QPO frequency, the sourcecount rate can differ by up to a factor of ~ 4, whereas at the same time therms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also findthat, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does notdepend upon which track the source occupies in the frequency vs. X-ray countrate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 areinconsistent with scenarios in which the properties of the kHz QPOs are onlydetermined by the mass accretion rate through the disk, whereas X-ray countrate also depends upon other sources of energy that do not affect the QPOs.