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. We find that for the lower kHz QPO both the slope of the best-fitting linear model to the time-lag spectrum and the total rms amplitude integrated over the 2–25 keV energy band 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 Comptonizing cloud or corona that is coupled to the innermost regions of the accretion disc, close to the neutron star.

中文翻译：

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中子星 X 射线双星中的千赫兹准周期振荡：滞后光谱随着光度的增加而变平

我们研究了一组中子星低质量 X 射线双星的千赫兹准周期振荡 (kHz QPO) 的能量相关时滞和 rms 分数幅度。我们发现，对于较低的 kHz QPO，最佳拟合线性模型对时滞谱的斜率和在 2-25 keV 能带上积分的总 rms 幅度都随着光源的光度呈指数下降。对于较高的 kHz QPO，时滞谱的斜率与零一致，而总 rms 幅度随光源的光度呈指数下降。我们表明，时滞谱的斜率和较低 kHz QPO 的总 rms 幅度都与 ~1 的斜率线性相关。最后，我们讨论了可能导致 kHz QPO 辐射特性的机制，