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The mass and radius of the neutron star (NS) in low-mass X-ray binaries canbe obtained by fitting the X-ray spectrum of the NS in quiescence, and the massand radius constrains the properties of dense matter in NS cores. A criticalingredient for spectral fits is the composition of the NS atmosphere: hydrogenatmospheres are assumed in most prior work, but helium atmospheres are possibleif the donor star is a helium white dwarf. Here we perform spectral fits toXMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13.This NS has the smallest inferred radius from previous spectral fitting.Assuming an atmosphere composed of hydrogen, we find a significantly largerradius, more consistent with those from other quiescent NSs. With a heliumatmosphere (an equally acceptable fit), we find even larger values for theradius.

Mass/Radius Constraints on the Quiescent Neutron Star in M13 Using Hydrogen and Helium Atmospheres