Measuring masses in low mass X-ray binaries via X-ray spectroscopy: the case of MXB 1659-298

TL;DR

This study demonstrates that high-resolution X-ray spectroscopy can effectively measure the radial velocity curve of the compact object in low-mass X-ray binaries, providing a new method for determining fundamental system parameters.

Contribution

The paper presents the first measurement of the radial velocity curve of the neutron star in MXB 1659-298 using X-ray spectroscopy, offering a novel approach for parameter estimation in such systems.

Findings

Measured the radial velocity semi-amplitude of the neutron star as 89±19 km/s.

Constrained the secondary star's mass to 0.3-0.8 solar masses.

Estimated the orbital inclination to be between 73° and 77°.

Abstract

The determination of fundamental parameters in low-mass X-ray binaries typically relies on measuring the radial velocity curve of the companion star through optical or near-infrared spectroscopy. It was recently suggested that high resolution X-ray spectroscopy might enable a measurement of the radial velocity curve of the compact object by monitoring the Doppler shifts induced by the orbital motion of the disc wind or the disc atmosphere. We analysed a Chandra-HETG+NuSTAR soft state observation of MXB 1659-298, an eclipsing neutron star low-mass X-ray binary (LMXB). We measured a radial velocity curve whose phase offset and semi-amplitude are consistent with the primary star. We derived the value for the semi-amplitude of the radial velocity for the compact object $K_1=89\pm19$ km s$^{-1}$, constrained the mass of the secondary ($0.3\leq M_2\leq0.8$ M$_\odot$) and the orbital inclination of the binary system ($73\leq i\leq77^\circ$). These values are consistent with previous estimates from independent methods. Via the same technique, the next generation of X-ray observatories equipped with high spectral resolution instruments (e.g., Athena) will have the potential to measure the radial velocity curve of the primary in high inclination X-ray binaries to an accuracy of a few per cent.