The drop of the coherence of the lower kHz QPOs is also observed in XTE J1701-462

TL;DR

This study analyzes the properties of lower kHz QPOs in XTE J1701-462, revealing a sharp drop in coherence and amplitude above ~800 Hz, consistent with the innermost stable circular orbit, and links QPO changes to accretion disk geometry.

Contribution

It provides the first detailed analysis of QPO coherence and amplitude across different source states in XTE J1701-462, supporting the ISCO interpretation and connecting QPO properties to disk scale height.

Findings

QPO quality factor peaks at ~800 Hz

QPO coherence and amplitude drop sharply above this frequency

QPO properties differ significantly between Z and atoll states

Abstract

We investigate the quality factor and RMS amplitude of the lower kHz QPOs from XTE J1701-462, a unique X-ray source which was observed in both the so-called Z and atoll states. Correcting for the frequency drift of the QPO, we show that, as in all sources for which such a correction can be applied, the quality factor and RMS amplitude drops sharply above above a critical frequency. For XTE J1701-462 this frequency is estimated to be ~800 Hz, where the quality factor reaches a maximum of ~200 (e.g. a value consistent with the one observed from more classical systems, such as 4U~1636-536). Such a drop has been interpreted as the signature of the innermost stable circular orbit, and that interpretation is consistent with the observations we report here. The kHz QPOs in the Z state are much less coherent and lower amplitude than they are in the atoll state. We argue that the change of the QPO properties between the two source states is related to the change of the scale height of the accretion disk; a prediction of the toy model proposed by barret et al. (2007). As a by-product of our analysis, we also increased the significance of the upper kHz QPO detected in the atoll phase up to 4.8 sigma (single trial significance), and show that the frequency separation (266.5+/-13.1 Hz) is comparable with the one measured from simultaneous twin QPOs the Z phase.