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We study the energy-dependent time lags and rms fractional amplitude of thekilohertz quasi-periodic oscillations (kHz QPOs) of a group of neutron-star lowmass X-ray binaries (LMXBs). We find that for the lower kHz QPO the slope ofthe best-fitting linear model to the time-lag spectrum and the total rmsamplitude integrated over the 2 to 25 keV energy band both decreaseexponentially with the luminosity of the source. For the upper kHz QPO theslope of the time-lag spectrum is consistent with zero, while the total rmsamplitude decreases exponentially with the luminosity of the source. We showthat both the slope of the time-lag spectrum and the total rms amplitude of thelower kHz QPO are linearly correlated with a slope of ~1. Finally, we discussthe mechanism that could be responsible for the radiative properties of the kHzQPOs, with the variability originating in a Comptonising cloud or corona thatis coupled to the innermost regions of the accretion disc, close to the neutronstar.

Kilohertz quasi-periodic oscillations in neutron-star X-ray binaries: Flattening of the lag spectrum with increasing luminosity