Investigation of the emission radii of kHz QPOs for the accreting millisecond X-Ray pulsars, Atoll and Z sources

TL;DR

This study estimates the emission locations of kHz QPOs in neutron star binaries using the Alfven wave oscillation model, revealing clustering around 16-19 km and similarities across different source types.

Contribution

It introduces a method to determine kHz QPO emission radii and compares these across various neutron star low mass X-ray binaries, highlighting commonalities.

Findings

Emission radii cluster around 16-19 km for most sources.

Cir X-1 has larger emission radii, possibly due to magnetic field effects.

Similar emission radii found in Atoll and Z sources, indicating shared magnetosphere-disk properties.

Abstract

We infer the emission positions of twin kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries (NS-LMXBs) based on the Alfven wave oscillation model (AWOM). For most sources, the emission radii of kHz QPOs cluster around a region of 16-19 km with the assumed NS radii of 15 km. Cir X-1 has the larger emission radii of 23-38 km than those of the other sources, which may be ascribed to its large magnetosphere-disk radius or strong NS surface magnetic field. SAX J1808.4-3658 is also a particular source with the relative large emission radii of kHz QPOs of 20 - 23 km, which may be due to its large inferred NS radius of 18 - 19 km. The emission radii of kHz QPOs for all the sources are larger than the NS radii, and the possible explanations of which are presented. The similarity of the emission radii of kHz QPOs (16-19 km) for both the low/high luminosity Atoll/Z sources is found, which indicates that both sources share the similar magnetosphere- disk radii.