Masses of Neutron Stars in High-Mass X-ray Binaries with Optical Astrometry

TL;DR

This paper demonstrates how microarcsecond optical astrometry can precisely measure neutron star masses in high-mass X-ray binaries, helping to constrain the neutron star equation of state.

Contribution

It provides detailed simulations showing that optical astrometry can accurately determine neutron star masses in HMXBs, improving constraints on their internal matter composition.

Findings

Mass measurements with 2.5% accuracy for X Per

Mass measurements with 6.5% accuracy for Vela X-1

Potential to constrain neutron star equations of state

Abstract

Determining the type of matter that is inside a neutron star (NS) has been a long-standing goal of astrophysics. Despite this, most of the NS equations of state (EOS) that predict maximum masses in the range 1.4-2.8 solar masses are still viable. Most of the precise NS mass measurements that have been made to date show values close to 1.4 solar masses, but a reliable measurement of an over-massive NS would constrain the EOS possibilities. Here, we investigate how optical astrometry at the microarcsecond level can be used to map out the orbits of High-Mass X-ray Binaries (HMXBs), leading to tight constraints on NS masses. While previous studies by Unwin and co-workers and Tomsick and co-workers discuss the fact that the future Space Interferometry Mission should be capable of making such measurements, the current work describes detailed simulations for 6 HMXB systems, including predicted constraints on all orbital parameters. We find that the direct NS masses can be measured to an accuracy of 2.5% (1-sigma) in the best case (X Per), to 6.5% for Vela X-1, and to 10% for two other HMXBs.