The kilo Hertz quasi-periodic oscillations in neutron star low-mass X-ray binaries as tori oscillation modes. I

TL;DR

This study models kHz QPOs in neutron star low-mass X-ray binaries as oscillation modes of relativistic thick tori, providing insights into their frequency relations and state transition behaviors.

Contribution

It introduces hydrodynamic simulations of relativistic tori to explain QPOs, linking observed frequencies to specific oscillation modes and state transitions.

Findings

Certain frequency pairs match observed QPO relations with a smaller neutron star mass.

Constant-$l$ tori cannot explain the full frequency range due to physical constraints.

The model accounts for frequency shifts during accretion disk state transitions.

Abstract

There have been many efforts to explain the dynamical mechanisms behind the phenomenology of quasi-periodic oscillations (QPOs) seen in the X-ray light curves of low-mass X-ray binaries. Up to now, none of the models can successfully explain all the frequencies observed in the power density spectrum of the light curve. Here we perform several hydrodynamic simulations of non-self-gravitating relativistic axisymmetric thick tori applied to the neutron star in the low-mass X-ray binary 4U 1636-53 and show how the observed oscillation modes triggered by different velocity perturbations give rise to a set of variability features similar to what we see in the observational X-ray data. When we match pairs of frequencies from the simulations of constant angular momentum ($l$) tori with the observed kilo Hertz QPOs, we find that certain combinations of frequencies lie on top of the observed relation, provided we assume a smaller mass for the neutron star than is generally assumed. However, constant-$l$ tori cannot match the entire range of frequencies observed for 4U 1636-53 due to physical constraints set by the torus size. We show that our model is consistent with the observed shift in QPO frequency that accompany state transitions of the accretion disk.