The Dense Matter Equation of State from Neutron Star Radius and Mass Measurements

TL;DR

This study uses advanced spectroscopic measurements and Bayesian analysis of twelve neutron stars to tightly constrain the dense matter equation of state, accounting for systematic uncertainties and recent theoretical effects.

Contribution

It introduces a comprehensive Bayesian framework incorporating systematic uncertainties and recent theoretical corrections to derive neutron star radii and constrain the dense matter equation of state.

Findings

Neutron star radii around 1.5 Msun are between 10.1 and 11.1 km.

Results suggest a weak contribution of three-body interactions in high-density nuclear matter.

Constraints are consistent with a relatively soft equation of state at high densities.

Abstract

We present a comprehensive study of spectroscopic radius measurements of twelve neutron stars obtained during thermonuclear bursts or in quiescence. We incorporate, for the first time, a large number of systematic uncertainties in the measurement of the apparent angular sizes, Eddington fluxes, and distances, in the composition of the interstellar medium, and in the flux calibration of X-ray detectors. We also take into account the results of recent theoretical calculations of rotational effects on neutron star radii, of atmospheric effects on surface spectra, and of relativistic corrections to the Eddington critical flux. We employ Bayesian statistical frameworks to obtain neutron star radii from the spectroscopic measurements as well as to infer the equation of state from the radius measurements. Combining these with the results of experiments in the vicinity of nuclear saturation density and the observations of ~2 Msun neutron stars, we place strong and quantitative constraints on the properties of the equation of state between approximately 2-8 times the nuclear saturation density. We find that around M=1.5 Msun, the preferred equation of state predicts radii between 10.1 - 11.1 km. When interpreting the pressure constraints in the context of high density equations of state based on interacting nucleons, our results suggest a relatively weak contribution of the three-body interaction potential.