Tidal deformability from GW170817 as a direct probe of the neutron star radius

TL;DR

This paper shows that the effective tidal deformability in neutron star mergers primarily depends on the chirp mass to radius ratio, enabling direct neutron star radius measurement from gravitational wave data.

Contribution

It introduces a new simplification linking tidal deformability to neutron star radius, allowing direct radius estimation from GW observations, independent of individual masses.

Findings

Effective tidal deformability is nearly mass-independent when chirp mass is fixed.

Upper limit on neutron star radius is approximately 13 km from GW170817 data.

Method can be applied to future neutron star mergers for radius measurements.

Abstract

Gravitational waves from the coalescence of two neutron stars were recently detected for the first time by the LIGO-Virgo collaboration, in event GW170817. This detection placed an upper limit on the effective tidal deformability of the two neutron stars and tightly constrained the chirp mass of the system. We report here on a new simplification that arises in the effective tidal deformability of the binary, when the chirp mass is specified. We find that, in this case, the effective tidal deformability of the binary is surprisingly independent of the component masses of the individual neutron stars, and instead depends primarily on the ratio of the chirp mass to the neutron star radius. Thus, a measurement of the effective tidal deformability can be used to directly measure the neutron star radius. We find that the upper limit on the effective tidal deformability from GW170817 implies that the radius cannot be larger than ~13km, at the 90% level, independent of the assumed masses for the component stars. The result can be applied generally, to probe the stellar radii in any neutron star-neutron star merger with a measured chirp mass. The approximate mass-independence disappears for neutron star-black hole mergers. Finally, we discuss a Bayesian inference of the equation of state that uses the measured chirp mass and tidal deformability from GW170817 combined with nuclear and astrophysical priors and discuss possible statistical biases in this inference.