Morphological analysis on the coherence of kHz QPOs

TL;DR

This paper analyzes the morphology of twin kHz QPOs in neutron star LMXBs, revealing distinct patterns in quality factors and frequencies between Z and Atoll sources, and exploring their implications for physical mechanisms.

Contribution

It provides a detailed morphological analysis of kHz QPOs, highlighting differences between source types and linking quality factors to spin frequencies and sidebands.

Findings

Quality factors of upper kHz QPOs are generally low and increase with frequency.

Distinct quality factor versus frequency patterns between Z and Atoll sources.

High quality factors in some Atoll sources correlate with higher spin frequencies.

Abstract

We take the recently published data of twin kHz quasi-period oscillations (QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and investigate the morphology of the samples, which focuses on the quality factor, peak frequency of kHz QPOs, and try to infer their physical mechanism. We notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in general) and increase with the kHz QPO peak frequencies for both Z and Atoll sources. (2) The distribution of quality factor versus frequency for the lower kHz QPOs are quite different between Z and Atoll sources. For most Z source samples, the quality factors of lower kHz QPOs are low (usually lower than 15) and rise steadily with the peak frequencies except for Sco X-1, which drop abruptly at the frequency of about 750 Hz. While for most Atoll sources, the quality factors of lower kHz QPOs are very high (from 2 to 200) and usually have a rising part, a maximum and an abrupt drop. (3) There are three Atoll sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality factors for lower kHz QPOs. These three sources have been detected with the spin frequencies and sidebands, in which the source with higher spin frequency presents higher quality factor of lower kHz QPOs and lower difference between sideband frequency and lower kHz QPO frequency.