Twin peak quasi-periodic oscillations as signature of oscillating cusp torus

TL;DR

This paper introduces a new model for high-frequency QPOs in neutron star binaries, involving oscillating cusp tori near the innermost stable orbit, which can fit observed data effectively.

Contribution

The paper proposes a novel oscillating cusp torus model for HF QPOs, providing a new explanation for observed frequencies and fitting data as well as or better than existing models.

Findings

Model predicts QPO frequencies as a function of a single parameter r_0.

Model fits observed data comparable to relativistic precession model.

Compatibility with neutron star rotation models demonstrated.

Abstract

Serious theoretical effort has been devoted to explain the observed frequencies of twin-peak quasi-periodic oscillations (HF QPOs) observed in low-mass X-ray neutron star binaries. Here we propose a new model of HF QPOs. Within its framework we consider an oscillating torus with cusp that changes location $r_0$ of its centre around radii very close to innermost stable circular orbit. The observed variability is assigned to global modes of accreted fluid motion that may give strong modulation of both accretion disc radiation and the accretion rate. For a given spacetime geometry, the model predicts that QPO frequencies are function of single parameter $r_0$. We illustrate that the model can provide fits of data comparable to those reached by other models, or even better. In particular it is compared to relativistic precession model. Moreover, we also illustrate that the model consideration is compatible with consideration of models of a rotating neutron star in the atoll source 4U~1636-53.