Neutron Star Masses and Radii from Quiescent Low-Mass X-ray Binaries

TL;DR

This study systematically analyzes neutron star radii from quiescent low-mass X-ray binaries, exploring how distance, absorption, and atmospheric composition affect measurements, and constrains the neutron star equation of state using Bayesian methods.

Contribution

It introduces a semi-analytic atmospheric model and applies Bayesian analysis to derive neutron star mass-radius relations considering various observational uncertainties.

Findings

Some absorption models are disfavored.

Hadronic matter is favored over exotic matter in neutron star composition.

Predicted neutron star radii align with current nuclear physics constraints.

Abstract

We perform a systematic analysis of neutron star radius constraints from five quiescent low-mass X-ray binaries and examine how they depend on measurements of their distances and amounts of intervening absorbing material, as well as their assumed atmospheric compositions. We construct and calibrate to published results a semi-analytic model of the neutron star atmosphere which approximates these effects for the predicted masses and radii. Starting from mass and radius probability distributions established from hydrogen-atmosphere spectral fits of quiescent sources, we apply this model to compute alternate sets of probability distributions. We perform Bayesian analyses to estimate neutron star mass-radius curves and equation of state (EOS) parameters that best-fit each set of distributions, assuming the existence of a known low-density neutron star crustal EOS, a simple model for the high-density EOS, causality, and the observation that the neutron star maximum mass exceeds $2~M_\odot$. We compute the posterior probabilities for each set of distance measurements and assumptions about absorption and composition. We find that, within the context of our assumptions and our parameterized EOS models, some absorption models are disfavored. We find that neutron stars composed of hadrons are favored relative to those with exotic matter with strong phase transitions. In addition, models in which all five stars have hydrogen atmospheres are found to be weakly disfavored. Our most likely models predict neutron star radii that are consistent with current experimental results concerning the nature of the nucleon-nucleon interaction near the nuclear saturation density.