Constraining the Mass and Radius of Neutron Stars in Globular Clusters

TL;DR

This study combines observations of neutron stars in globular clusters to constrain their mass-radius relationship and the dense matter equation of state, highlighting the impact of various uncertainties and model assumptions.

Contribution

It provides the first combined analysis of multiple neutron star systems to derive constraints on their radii and dense matter properties, considering systematic uncertainties.

Findings

Neutron star radius likely between 10-14 km for 1.4 solar masses

Strong phase transitions in dense matter are favored by the data

Radii smaller than 12 km are preferred under certain model assumptions

Abstract

We analyze observations of eight quiescent low-mass X-ray binaries in globular clusters and combine them to determine the neutron star mass-radius curve and the equation of state of dense matter. We determine the effect that several uncertainties may have on our results, including uncertainties in the distance, the atmosphere composition, the neutron star maximum mass, the neutron star mass distribution, the possible presence of a hotspot on the neutron star surface, and the prior choice for the equation of state of dense matter. We find that the radius of a 1.4 solar mass neutron star is most likely from 10 to 14 km and that tighter constraints are only possible with stronger assumptions about the nature of the neutron stars, the systematics of the observations, or the nature of dense matter. Strong phase transitions are preferred over other models and interpretations of the data with a Bayes factor of 8 or more, and in this case, the radius is likely smaller than 12 km. However, radii larger than 12 km are preferred if the neutron stars have uneven temperature distributions.