Improved Mass and Radius Constraints for Quiescent Neutron Stars in Omega Cen and NGC 6397

TL;DR

This study uses X-ray and optical observations of neutron stars in two globular clusters to refine their mass and radius estimates, challenging previous claims of unusually small neutron star radii.

Contribution

It provides improved constraints on neutron star radii by analyzing multiple systematic factors and combining X-ray and optical data, offering more accurate measurements.

Findings

Neutron star radii are consistent with 10-13 km range.

Systematic effects significantly influence mass-radius inferences.

Previous claims of very small radii are not supported by this analysis.

Abstract

We use Chandra and XMM observations of the globular clusters $\omega$ Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favored by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.