On obtaining neutron-star mass and radius constraints from quiescent low-mass X-ray binaries in the Galactic plane

TL;DR

This study assesses the potential of using X-ray spectra from quiescent low-mass X-ray binaries in the Galactic plane to constrain neutron star masses and radii, highlighting current limitations and future prospects with improved data.

Contribution

It demonstrates the feasibility and challenges of measuring neutron star radii in the Galactic plane and explores how higher quality spectra can improve these constraints.

Findings

Only two sources provided useful radius constraints.

Omitting spectral components biases radius estimates.

Simulated Athena data can reduce systematic biases.

Abstract

X-ray spectral analysis of quiescent low-mass X-ray binaries (LMXBs) has been one of the most common tools to measure the radius of neutron stars (NSs) for over a decade. So far, this method has been mainly applied to NSs in globular clusters, primarily because of their well-constrained distances. Here, we study Chandra data of seven transient LMXBs in the Galactic plane in quiescence to investigate the potential of constraining the radius (and mass) of the NSs inhabiting these systems. We find that only two of these objects had X-ray spectra of sufficient quality to obtain reasonable constraints on the radius, with the most stringent being an upper limit of $R\lesssim$14.5 km for EXO 0748-676 (for assumed ranges for mass and distance). Using these seven sources, we also investigate systematic biases on the mass/radius determination; for Aql X-1 we find that omitting a power-law spectral component when it does not seem to be required by the data, results in peculiar trends in the obtained radius with changing mass and distance. For EXO 0748-676 we find that a slight variation in the lower limit of the energy range chosen for the fit leads to systematically different masses and radii. Finally, we simulated Athena spectra and found that some of the biases can be lifted when higher quality spectra are available and that, in general, the search for constraints on the equation of state of ultra-dense matter via NS radius and mass measurements may receive a considerable boost in the future.