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