  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.
