Mechanical model of a boundary layer for the parallel tracks of kilohertz quasi-periodic oscillations in accreting neutron stars

TL;DR

This paper proposes a simple boundary layer model for neutron star accretion that explains the parallel tracks phenomenon in kilohertz QPOs by balancing spin-up and spin-down processes, matching observed variability timescales.

Contribution

It introduces a novel boundary layer model that accounts for the parallel tracks effect in kHz QPOs, linking the phenomena to the interaction timescales of mass and angular momentum transfer.

Findings

Model reproduces the parallel tracks effect in QPO frequencies.

Time scales for mass and angular momentum transfer are similar.

Explains variability at hours and days timescales.

Abstract

Kilohertz-scale quasi-periodic oscillations (kHz QPOs) are a distinct feature of the variability of neutron star low-mass X-ray binaries. Among all the variability modes, they are especially interesting as a probe for the innermost parts of the accretion flow, including the accretion boundary layer (BL) on the surface of the neutron star. All the existing models of kHz QPOs explain only part of their rich phenomenology. Here, we show that some of their properties may be explained by a very simple model of the BL that is spun up by accreting rapidly rotating matter from the disk and spun down by the interaction with the neutron star. In particular, if the characteristic time scales for the mass and the angular momentum transfer from the BL to the star are of the same order of magnitude, our model naturally reproduces the so-called parallel tracks effect, when the QPO frequency is correlated with luminosity at time scales of hours but becomes uncorrelated at time scales of days. The closeness of the two time scales responsible for mass and angular momentum exchange between the BL and the star is an expected outcome of the radial structure of the BL.