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

TL;DR

This study analyzes the energy dependence and origin of kHz QPOs in Sco X-1 using RXTE and Insight-HXMT data, revealing their energy band, amplitude behavior, and likely origin in Comptonization within the transition layer.

Contribution

First detailed spectral-timing analysis of Sco X-1's kHz QPOs across multiple X-ray observatories, identifying their energy range and origin.

Findings

kHz QPOs are detectable in specific energy bands for RXTE and Insight-HXMT.

The fractional rms of QPOs increases with energy then plateaus around 16-20 keV.

QPOs likely originate from Comptonization in the transition layer, not the accretion disk or neutron star surface.

Abstract

We present a detailed spectral-timing analysis of the Kilohertz quasi-periodic oscillations (kHz QPOs) in Sco X-1 using the data of Rossi X-ray Timing Explorer ($RXTE$) and the Hard X-ray Modulation Telescope ($Insight$-HXMT). The energy band with detectable kHz QPOs is studied for the first 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 normal branch, the energy band is narrower. The fractional root mean square (rms) of the kHz QPOs increases with energy at lower energy, reaches a plateau at about 16 keV and 20 keV for the lower and upper peaks, and then levels off though with a large uncertainty. The simulation of the deadtime effect of $RXTE$/PCA shows that the deadtime does not affect much the search of the kHz QPOs but makes 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 not originate from the black body emission of the accretion disk and neutron star surface. In addition, with the combined analysis of the energy spectra and the absolute rms spectra of kHz QPOs, we suggest that the kHz QPOs in Sco X-1 originate from the Comptonization of the inner part of the transition layer, where the rotation sets the frequency and the inward bulk motion makes the spectrum harder.