KHz QPOs in LMXBs, relations between different frequencies and compactness of stars

TL;DR

This paper estimates the masses of four compact stars in Low-Mass X-ray Binaries by analyzing their observed kHz QPOs, linking stellar radius to the marginally stable orbit predicted by general relativity.

Contribution

It introduces a novel method to determine stellar masses from kHz QPOs and provides the first such estimates for these stars.

Findings

Masses of four compact stars are estimated from kHz QPO data.

Stellar radii are close to the marginally stable orbit predicted by GR.

First mass estimates derived from observed kHz QPOs for these stars.

Abstract

We suggest that the mass of four compact stars SAX J1808.4$-$3658, KS 1731$-$260, SAX J1750.8$-$2900 and IGR J17191$-$2821 can be determined from the difference in the observed kiloHertz quasi periodic oscillations (kHz QPO-s) of these stars. The stellar radius is very close to the marginally stable orbit $R_{ms}$ as predicted by Einstein's general relativity. It may be noted that the first of these stars was suggested to be a strange star more than a decade back by Li \emph{et al.} (1999) from the unique millisecond X-ray pulsations with an accurate determination of its rotation period. It showed kHz QPO-s eight years back and so far it is the only set that has been observed. This is the first time we give an estimate of the mass of the star and of three other compact stars in Low-Mass X-ray Binaries using their observed kHz QPO-s.