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We present a detailed spectral-timing analysis of the Kilohertzquasi-periodic oscillations (kHz QPOs) in Sco X-1 using the data of Rossi X-rayTiming Explorer ($RXTE$) and the Hard X-ray Modulation Telescope($Insight$-HXMT). The energy band with detectable kHz QPOs is studied for thefirst time: on the horizontal branch, it is $\sim$ 6.89--24.01 keV and $\sim$8.68--21.78 keV for the upper and lower kHz QPOs detected by $RXTE$, and $\sim$9--27.5 keV for the upper kHz QPOs by $Insight$-HXMT; on the lower normalbranch, the energy band is narrower. The fractional root mean square (rms) ofthe kHz QPOs increases with energy at lower energy, reaches a plateau at about16 keV and 20 keV for the lower and upper peaks, and then levels off thoughwith a large uncertainty. The simulation of the deadtime effect of $RXTE$/PCAshows that the deadtime does not affect much the search of the kHz QPOs butmakes the rms amplitude underestimated. No significant QPO is detected below$\sim$ 6 keV as shown by the $RXTE$ data, implying that the kHz QPOs do notoriginate from the black body emission of the accretion disk and neutron starsurface. In addition, with the combined analysis of the energy spectra and theabsolute rms spectra of kHz QPOs, we suggest that the kHz QPOs in Sco X-1originate from the Comptonization of the inner part of the transition layer,where the rotation sets the frequency and the inward bulk motion makes thespectrum harder.

Study on the energy limits of kHz QPOs in Sco X-1 with $RXTE$ and $Insight$-HXMT observations