# Timescale of twin-peak quasi-periodic oscillations and mass of accreting neutron stars

## TL;DR

This paper investigates the relationship between QPO periods in neutron star systems and their masses, suggesting that characteristic timescales can inform on neutron star mass and equation of state.

## Contribution

It provides a comparative analysis of QPO timescales across multiple neutron star systems, linking these to neutron star mass estimates and constraining models of their internal structure.

## Key findings

- XTE J1807.4-294 has a longer QPO timescale than other neutron star LMXBs.
- Models imply the neutron star in XTE J1807.4-294 is about 50% more massive.
- QPO characteristics can help distinguish between different neutron star equations of state.

## Abstract

Einstein's general relativity predicts that orbital motion of accreted gas approaching a neutron star (NS) in a NS low-mass X-ray binary (LMXB) system occurs on a time scale proportional to the NS mass. Radiation of the gas accounts for most of the observed LMXBs variability. In more than a dozen of sources twin-peak quasi-periodic oscillations (QPOs) have been observed. Inspired by the expected proportionality between periods of orbital motion and NS mass we present a straightforward comparison among these sources. We investigate relations between QPO periods and their ratios and identify characteristic time scales of QPOs associated to individual sources. These timescales are likely determined by the relative mass of each NS. We show that the characteristic time scale of the millisecond pulsar XTE J1807.4-294 is longer than for most other NS LMXBs. Consequently, models of QPOs that consider geodesic orbital frequencies imply that the X-ray pulsars' mass has to be about 50 % higher than the average mass of other sources. Consideration of other X-ray pulsars indicates that the exceptionality of XTE J1807.4-294 cannot be related to NS magnetic field in any simple manner. We suggest that QPOs observed in this source can help to discriminate between the proposed versions of the NS equation of state.