Highly coherent kilohertz quasi-periodic oscillations from a neutron star X-ray binary EXO 1745-248

TL;DR

This paper reports the discovery of a highly coherent kilohertz QPO at 690 Hz from a neutron star binary, with a record-high Q-factor, and discusses implications for QPO models and coherence measurement techniques.

Contribution

The study presents the first observation of an exceptionally high Q-factor in a kHz QPO and analyzes its variability, challenging existing models and the effectiveness of the shift-and-add technique.

Findings

Record-high Q-factor of 264.5 for a kHz QPO.

High Q-factors can be observed over short durations.

Shift-and-add technique may underestimate maximum Q-factors.

Abstract

We report the discovery ($20\sigma$) of kilohertz quasi-periodic oscillations (kHz QPOs) at ~ 690 Hz from the transient neutron star low-mass X-ray binary EXO 1745-248. We find that this is a lower kHz QPO, and systematically study the time variation of its properties using smaller data segments with and without the shift-and-add technique. The quality (Q) factor occasionally significantly varies within short ranges of frequency and time. A high Q-factor (264.5 +- 38.5) of the QPO is found for a 200 s time segment, which might be the largest value reported in the literature. We argue that an effective way to rule out kHz QPO models is to observationally find such high Q-factors, even for a short duration, as many models cannot explain a high coherence. However, as we demonstrate, the shift-and-add technique cannot find a very high Q-factor which appears for a short period of time. This shows that the coherences of kHz QPOs can be higher than the already high values reported using this technique, implying further constraints on models. We also discuss the energy dependence of fractional rms amplitude and Q-factor of the kHz QPO.