New constraints on radii and tidal deformabilities of neutron stars from GW170817

TL;DR

This paper constrains neutron star radii and tidal deformabilities using GW170817 data, exploring a vast range of equations of state, including phase transitions, to statistically determine the most probable stellar properties.

Contribution

It introduces a comprehensive analysis of one million EOSs with and without phase transitions, applying GW170817 constraints to statistically infer neutron star radii and deformabilities.

Findings

Neutron star radius at 1.4 solar masses is constrained to 12.00-13.45 km.

Minimum tidal deformability at 1.4 solar masses is greater than 375.

Detection of certain binary parameters can rule out twin-star solutions.

Abstract

We explore in a parameterized manner a very large range of physically plausible equations of state (EOSs) for compact stars for matter that is either purely hadronic or that exhibits a phase transition. In particular, we produce two classes of EOSs with and without phase transitions, each containing one million EOSs. We then impose constraints on the maximum mass, ($M < 2.16 M_{\odot}$), and on the dimensionless tidal deformability ($\tilde{\Lambda} <800$) deduced from GW170817, together with recent suggestions of lower limits on $\tilde{\Lambda}$. Exploiting more than $10^9$ equilibrium models for each class of EOSs, we produce distribution functions of all the stellar properties and determine, among other quantities, the radius that is statistically most probable for any value of the stellar mass. In this way, we deduce that the radius of a purely hadronic neutron star with a representative mass of $1.4\,M_{\odot}$ is constrained to be $12.00\!<\!R_{1.4}/{\rm km}\!<\!13.45$ at a $2$-$\sigma$ confidence level, with a most likely value of $\bar{R}_{1.4}=12.39\,{\rm km}$; similarly, the smallest dimensionless tidal deformability is $\tilde{\Lambda}_{1.4}\!>\!375$, again at a $2$-$\sigma$ level. On the other hand, because EOSs with a phase transition allow for very compact stars on the so-called `twin-star' branch, small radii are possible with such EOSs although not probable, i.e. $8.53\!<\!R_{1.4}/{\rm km}\!<\!13.74$ and $\bar{R}_{1.4}=13.06\,{\rm km}$ at a $2$-$\sigma$ level, with $\tilde{\Lambda}_{1.4}\!>\!35.5$ at a $3$-$\sigma$ level. Finally, since these EOSs exhibit upper limits on $\tilde{\Lambda}$, the detection of a binary with total mass of $3.4\,M_{\odot}$ and $\tilde{\Lambda}_{1.7}\!>\!461$ can rule out twin-star solutions.