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

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.

10 pages, 7 figures. Accepted for publication in MNRAS

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• X-ray Spectral and Timing evolution of MAXI J1727-203 with NICER
• A Comprehensive X-ray Report on AT2019wey
• High-frequency variability in neutron-star low-mass X-ray binaries
• The amplitude of the kilohertz quasi-periodic oscillations in 4U 1636$-$53 in the frequency-energy space
• A Machine Learning Approach For Classifying Low-mass X-ray Binaries Based On Their Compact Object Nature
• Innermost stable circular orbits of neutron stars in dilatonic-Einstein-Gauss-Bonnet theory
• Lense-Thirring precession of neutron-star accretion flows: Relativistic versus classical precession
• A NICER and AstroSat view of the neutron star low-mass X-ray binary 1A 1246-588