Oscillations of relativistic axisymmetric tori and implications for modelling kHz-QPOs in neutron-star X-ray binaries

TL;DR

This study analyzes the oscillation modes of relativistic tori around neutron stars to explain observed high-frequency QPOs in X-ray binaries, extending previous models to more realistic spacetime geometries.

Contribution

It introduces a global eigenvalue analysis of axisymmetric p-modes in relativistic tori within Hartle-Thorne spacetime, considering various angular momentum distributions.

Findings

p-modes can explain most observed high-frequency QPOs in certain sources.

The model accounts for the linear relation between upper and lower QPO frequencies at high frequencies.

At lower frequencies, p-modes do not reproduce the observed frequency relations.

Abstract

We perform a global linear perturbative analysis, and investigate the oscillation properties of relativistic, non-selfgravitating tori orbiting around neutron stars in the slow rotation limit approximation. Extending the work done in Schwarzschild and Kerr backgrounds, we consider the axisymmetric oscillations of vertically integrated tori in the Hartle-Thorne spacetime. The equilibrium models are constructed by selecting a number of different non-Keplerian distributions of specific angular momentum, allowing for disc sizes $L \sim 0.5 - 600$ gravitational radii. Our results, obtained after solving a global eigenvalue problem to compute the xisymmetric $p$-modes, indicate that such oscillation modes could account with most observed lower ($\nu_L$) and upper ($\nu_U$) high frequency quasi-periodic oscillations for Sco X-1, and for some Z sources and Atoll sources with $\nu_L\ gtrsim 500$ Hz. However, when $\nu_L \lesssim 500$ Hz, $p$-modes oscillations do not account for the linear relation $\nu_U = A \nu_L + B$, $B \neq 0$ between the upper and the lower high frequency quasi-periodic oscillations that are observed in neutron star low-mass X-ray binaries.