The highest-frequency kHz QPOs in neutron star low mass X-ray binaries

TL;DR

This study analyzes the highest-frequency kHz QPOs in neutron star low mass X-ray binaries, providing constraints on neutron star properties and supporting orbital motion models within general relativity.

Contribution

It reports the highest lower limit on kHz QPO frequency to date and demonstrates that observed frequencies are consistent with models of rotating neutron stars and the ISCO, constraining neutron star mass and radius.

Findings

Highest-frequency kHz QPO lower limit is 1267 Hz.

Neutron star mass in 4U 0614+09 is constrained to less than 2.1 solar masses.

Neutron star radii are constrained within narrow ranges for multiple sources.

Abstract

We investigate the highest-frequency kHz QPOs previously detected with RXTE in six neutron star (NS) low mass X-ray binaries. We find that the highest-frequency kHz QPO detected in 4U 0614+09 has a 1267 Hz 3$\sigma$ confidence lower limit on its centroid frequency. This is the highest such limit reported to date, and of direct physical interest as it can be used to constrain QPO models and the supranuclear density equation of state (EoS). We compare our measured frequencies to maximum orbital frequencies predicted in full GR using models of rotating neutron stars with a number of different modern EoS and show that these can accommodate the observed QPO frequencies. Orbital motion constrained by NS and ISCO radii is therefore a viable explanation of these QPOs. In the most constraining case of 4U 0614+09 we find the NS mass must be M$<$2.1 M$_{\odot}$. From our measured QPO frequencies we can constrain the NS radii for five of the six sources we studied to narrow ranges ($\pm$0.1-0.7 km) different for each source and each EoS.