High Mass X-ray Binaries in the NIR : Orbital solutions of two highly obscured systems

TL;DR

This paper presents the first orbital solutions for two highly obscured eclipsing X-ray binary systems using NIR spectroscopy, providing new measurements of neutron star and companion star masses.

Contribution

It introduces a novel method of using NIR spectroscopy to determine orbital parameters and masses in obscured XRB systems, expanding the tools for neutron star mass measurements.

Findings

First accurate neutron star and companion masses in these systems

Spectral classification of donor stars using NIR spectroscopy

Explanation of spin period evolution mechanisms

Abstract

The maximum mass of a neutron star (NS) is poorly defined. Theoretical attempts to define this mass have thus far been unsuccessful. Observational results currently provide the only means of narrowing this mass range down. Eclipsing X-ray binary (XRB) pulsar systems are the only interacting binaries in which the mass of the NS may be measured directly. Only 10 such systems are known to exist, 6 of which have yielded NS masses in the range 1.06 - 1.86 M$_{\odot}$.We present the first orbital solutions of two further eclipsing systems, OAO 1657-415 and EXO 1722-363, whose donor stars have only recently been identified. Using observations obtained using the VLT/ISAAC NIR spectrograph, our initial work was concerned with providing an accurate spectral classification of the two counterpart stars, leading to a consistent explanation of the mechanism for spin period evolution of OAO 1657-415. Calculating radial velocities allowed orbital solutions for both systems to be computed. These are the first accurate determinations of the NS and counterpart masses in XRB pulsar systems to be made employing NIR spectroscopy.