Constraining the mass and moment of inertia of neutron stars from quasi-periodic oscillations in X-ray binaries

Abstract

Neutron stars are the densest objects known in the Universe. Being the final product of stellar evolution, their internal composition and structure is rather poorly constrained by measurements. It is the purpose of this paper to put some constrains on the mass and moment of inertia of neutron stars based on the interpretation of kHz quasi-periodic oscillations observed in low mass X-ray binaries. We use observations of high-frequency quasi-periodic observations (HF-QPOs) in low mass X-ray binaries (LMXBs) to look for the average mass and moment of inertia of neutron stars. This is done by applying our parametric resonance model to discriminate between slow and fast rotators. We fit our model to data from ten LMXBs for which HF-QPOs have been seen and the spin of the enclosed accreting neutron star is known. For a simplified analysis we assume that all neutron stars possess the same properties (same mass $M_*$ and same moment of inertia $I_*$). We find an average mass $M_* \approx 2.0-2.2\, M_{\odot}$. The corresponding average moment of inertia is then $I_* \approx 1-3 \times 10^{38}\;{\rm kg\,m^2} \approx 0.5-1.5 \, (10\;\textrm{ km})^2 \, M_\odot$ which equals to dimensionless spin parameter $\tilde{a} \approx 0.05-0.15$ for slow rotators (neutron stars with a spin frequency roughly about 300~Hz) respectively $\tilde{a} \approx 0.1-0.3$ for fast rotators (neutron stars with the spin frequency roughly about 600~Hz).