Inferring neutron star properties from GW170817 with universal relations

Because all neutron stars share a common equation of state, tidal deformability constraints from the compact binary coalescence GW170817 have implications for the properties of neutron stars in other systems. Using equation-of-state insensitive relations between macroscopic observables like moment of inertia (I ), tidal deformability (Λ ) and stellar compactness, we derive constraints on these properties as a function of neutron star mass based on the LIGO-Virgo Collaboration's canonical deformability measurement, Λ_1.4=19 0_-120⁺³⁹⁰. Specific estimates of Λ , I , dimensionless spin χ , and stellar radius R for a few systems targeted by radio or x-ray studies are extracted from the general constraints. We also infer the canonical neutron star radius as R_1.4=10.9_-1.5^+1.9 km at 90% confidence. We further demonstrate how a gravitational-wave measurement of Λ_1.4 can be combined with independent measurements of neutron star radii to tighten constraints on the tidal deformability as a proxy for the equation of state. We find that GW170817 and existing observations of six thermonuclear bursters in low-mass x-ray binaries jointly imply Λ_1.4=19 6_-63⁺⁹² at the 90% confidence level.