Parallel tracks as quasi-steady states for the magnetic boundary layers in neutron-star low-mass X-ray binaries

TL;DR

This paper models the magnetic boundary layer in neutron-star low-mass X-ray binaries to explain the observed parallel tracks in kHz QPO frequency versus flux, revealing quasi-steady states influenced by boundary layer dynamics.

Contribution

It introduces a boundary region model for kHz QPOs that reproduces parallel tracks and explains their long-term evolution in neutron-star LMXBs.

Findings

Parallel tracks correspond to quasi-steady states of the boundary layer.

Radial width of the boundary layer varies with flux evolution.

A generic model describes the average steady behavior of the boundary region.

Abstract

The neutron stars in low-mass X-ray binaries (LMXBs) are usually thought to be weakly magnetized objects accreting matter from their low-mass companions in the form of a disk. Albeit weak as compared to those in young neutron-star systems, the neutron-star magnetospheres in LMXBs can play an important role in determining the correlations between spectral and temporal properties. Parallel tracks appearing in the plane of kilohertz (kHz) quasi-periodic oscillation (QPO) frequency versus X-ray flux can be used as a tool to study the magnetosphere-disk interaction in neutron-star LMXBs. For dynamically important weak fields, the formation of a non-Keplerian magnetic boundary layer at the innermost disk truncated near the surface of the neutron star is highly likely. Such a boundary region may harbor oscillatory modes of frequencies in the kHz range. We generate parallel tracks using the boundary region model of kHz QPOs. We also present the direct application of our model to the reproduction of the observed parallel tracks of individual sources such as 4U 1608--52, 4U 1636--53, and Aql X-1. We reveal how the radial width of the boundary layer must vary in the long-term flux evolution of each source to regenerate the parallel tracks. The run of the radial width looks similar for different sources and can be fitted by a generic model function describing the average steady behavior of the boundary region in the long term. The parallel tracks then correspond to the possible quasi-steady states the source can occupy around the average trend.