MASSES OF NEUTRON STARS IN HIGH-MASS X-RAY BINARIES WITH OPTICAL ASTROMETRY

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 M(circle dot) are still viable. Most of the precise NS mass measurements that have been made to date show values close to 1.4 M(circle dot), but a reliable measurement of an overmassive 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 coworkers and Tomsick and coworkers discuss the fact that the future Space Interferometry Mission should be capable of making such measurements, the current work describes detailed simulations for six HMXB systems, including predicted constraints on all orbital parameters. We find that the direct NS masses can be measured to an accuracy of similar to 2.5% (1 sigma) in the best case (X Per), to similar to 6.5% for Vela X-1, and to similar to 10% for two other HMXBs.