The radius of the quiescent neutron star in the globular cluster M13

TL;DR

This study uses X-ray spectra from multiple observatories to precisely measure the radius of a quiescent neutron star in M13, providing tighter constraints on dense matter equations of state.

Contribution

First to combine multi-mission X-ray data for improved neutron star radius measurement in M13, considering atmosphere composition and distance uncertainties.

Findings

Radius estimated at ~12.2 km with hydrogen atmosphere

Inclusion of distance uncertainty slightly broadens radius estimate

Helium atmosphere implies a significantly larger radius

Abstract

X-ray spectra of quiescent low-mass X-ray binaries containing neutron stars can be fit with atmosphere models to constrain the mass and the radius. Mass-radius constraints can be used to place limits on the equation of state of dense matter. We perform fits to the X-ray spectrum of a quiescent neutron star in the globular cluster M13, utilizing data from ROSAT, Chandra and XMM-Newton, and constrain the mass-radius relation. Assuming an atmosphere composed of hydrogen and a 1.4${\rm M}_{\odot}$ neutron star, we find the radius to be $R_{\rm NS}=12.2^{+1.5}_{-1.1}$ km, a significant improvement in precision over previous measurements. Incorporating an uncertainty on the distance to M13 relaxes the radius constraints slightly and we find $R_{\rm NS}=12.3^{+1.9}_{-1.7}$ km (for a 1.4${\rm M}_{\odot}$ neutron star with a hydrogen atmosphere), which is still an improvement in precision over previous measurements, some of which do not consider distance uncertainty. We also discuss how the composition of the atmosphere affects the derived radius, finding that a helium atmosphere implies a significantly larger radius.