Resonant radii of kHz quasi-periodic oscillations in Keplerian discs orbiting neutron stars

TL;DR

This paper investigates the resonant radii of kHz QPOs in neutron star accretion disks, analyzing gravitational potential effects and non-linear mode interactions to explain observed phenomena and implications for neutron star mass estimates.

Contribution

It provides a detailed analysis of the gravitational and non-linear effects at resonant radii, offering new insights into QPO behavior and neutron star mass estimations.

Findings

Resonant radii influence QPO amplitude variations.

Higher-order gravitational terms affect non-linear mode dynamics.

Potential to revise neutron star mass estimates based on QPO frequencies.

Abstract

Exchange of dominance between twin kHz quasi-periodic oscillations (QPOs) observed in some low-mass-X-ray-binaries (LMXB) suggests the possibility of a resonance between two oscillatory modes. We study the behaviour of the effective gravitational potential around specific resonant radii, and estimate the role of the higher-order terms governing the non-linear, anharmonic forcing. We discuss the impact it has on the mode amplitude in the linear and non--linear regimes. We also discuss a related possibility of lowering of the neutron star mass estimates from the highest observed QPO frequencies.