Is there a link between the neutron-star spin and the frequency of the kilohertz quasi-periodic oscillations?

TL;DR

This paper reevaluates the assumed link between neutron-star spin and kHz QPO frequencies, suggesting they may not be directly related, challenging previous interpretations based on observed frequency ratios.

Contribution

The study critically examines existing data and proposes that the correlation between neutron-star spin and kHz QPOs may not be as direct as previously thought.

Findings

Data does not conclusively support a direct link between $ u_s$ and $\Delta u$

Frequency ratios cluster around 0.5 and 1, but correlation with spin is uncertain

Proposes methods to test the independence of these quantities

Abstract

There is a general consensus that the frequencies of the kilohertz Quasi-Periodic Oscillations (kHz QPOs) in neutron-star low-mass X-ray binaries are directly linked to the spin of the neutron star. The root of this idea is the apparent clustering of the ratio of the frequency difference of the kHz QPOs and the neutron-star spin frequency, $\Delta\nu/\nu_s$, at around 0.5 and 1 in ten systems for which these two quantities have been measured. Here we reexamine all available data of sources for which there exist measurements of two simultaneous kHz QPOs and spin frequencies, and we advance the possibility that $\Delta\nu$ and $\nu_s$ are not related to each other. We discuss ways in which this possibility could be tested with current and future observations.