Inferring neutron star properties from GW170817 with universal relations

Abstract: 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 ($\Lambda$) 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, $\Lambda_{1.4} = 190^{+390}_{-120}$. Specific estimates of $\Lambda$, $I$, dimensionless spin $\chi$, 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 $R_{1.4} = 10.9^{+1.9}_{-1.5}$ km at 90$\%$ confidence. We further demonstrate how a gravitational-wave measurement of $\Lambda_{1.4}$ 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 $\Lambda_{1.4} = 196^{+92}_{-63}$ at the 90$\%$ confidence level.