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

X-ray spectra of quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars can be fit with atmosphere models to constrain the mass and the radius. Mass-radius constraints can then be used to place limits on the equation of state of dense matter. However, derived radius constraints depend heavily on the distance to the neutron star, a quantity which is often difficult to constrain without parallax measurements. We therefore turn to qLMXBs in globular clusters, whose distances are known to within ∼5-10%. 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 1.4M_{⊙} neutron star, we find the radius to be 12.2^{+1.5}_{-1.1} km, a significant improvement in precision over previous measurements. I will discuss how the composition of the atmosphere affects the derived radius, finding that a helium atmosphere implies a significantly larger radius. I will also discuss the results in the context of the recent binary neutron star merger, GW170817, which provided limits on the tidal deformability of neutron stars, in turn implying limits on the neutron star radius that are in agreement with our measurements for M13.