The Magneto Hydro Dynamical Model of KHz Quasi Periodic Oscillations in Neutron Star Low Mass X-ray Binaries (II)

TL;DR

This paper introduces a magnetohydrodynamics model considering the compressed magnetosphere to explain kHz QPOs in neutron star LMXBs, aligning well with observed data and revealing how QPO frequencies relate to accretion rates.

Contribution

The study extends previous MHD models by incorporating magnetosphere compression, providing a better explanation of QPO frequencies and their dependence on accretion rates in neutron star systems.

Findings

Model agrees with observations of six sources with measured spins.

kHz QPOs originate from MHD waves in the compressed magnetosphere.

Twin kHz QPOs are produced in different parts of the accretion disk.

Abstract

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries (LMXBs) with a new magnetohydrodynamics (MHD) model, in which the compressed magnetosphere is considered. The previous MHD model (Shi \& Li 2009) is re-examined and the relation between the frequencies of the kHz QPOs and the accretion rate in LMXBs is obtained. Our result agrees with the observations of six sources (4U 0614+09, 4U 1636--53, 4U 1608--52, 4U 1915--15, 4U 1728--34, XTE 1807--294) with measured spins. In this model the kHz QPOs originate from the MHD waves in the compressed magnetosphere. The single kHz QPOs and twin kHz QPOs are produced in two different parts of the accretion disk and the boundary is close to the corotation radius. The lower QPO frequency in a frequency-accretion rate diagram is cut off at low accretion rate and the twin kHz QPOs encounter a top ceiling at high accretion rate due to the restriction of innermost stable circular orbit.