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 ($\mathrm{\ensuremath{\Lambda}}$) 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, ${\mathrm{\ensuremath{\Lambda}}}_{1.4}=19{0}_{\ensuremath{-}120}^{+390}$. Specific estimates of $\mathrm{\ensuremath{\Lambda}}$, $I$, dimensionless spin $\ensuremath{\chi}$, 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 ${R}_{1.4}={10.9}_{\ensuremath{-}1.5}^{+1.9}\text{ }\text{ }\mathrm{km}$ at 90% confidence. We further demonstrate how a gravitational-wave measurement of ${\mathrm{\ensuremath{\Lambda}}}_{1.4}$ 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 ${\mathrm{\ensuremath{\Lambda}}}_{1.4}=19{6}_{\ensuremath{-}63}^{+92}$ at the 90% confidence level.