GW190814: Impact of a 2.6 solar mass neutron star on nucleonic equations of state

TL;DR

This study uses covariant density functional theory to explore whether a 2.6 solar mass compact object could be a neutron star, but findings suggest it is more likely a black hole due to equation of state constraints.

Contribution

The paper demonstrates how tuning energy density functionals within covariant density functional theory can model super-massive neutron stars and tests their consistency with observational constraints.

Findings

Stiffening the equation of state to support 2.6 Msun neutron stars conflicts with heavy-ion collision constraints.

Models that support such massive neutron stars are inconsistent with low deformability measurements.

The 2.6 Msun object is likely the lightest black hole, not a neutron star.

Abstract

Is the secondary component of GW190814 the lightest black hole or the heaviest neutron star ever discovered in a double compact-object system [R. Abbott et al., ApJ Lett., 896, L44 (2020)]? This is the central question animating this letter. Covariant density functional theory provides a unique framework to investigate both the properties of finite nuclei and neutron stars, while enforcing causality at all densities. By tuning existing energy density functionals we were able to: (a) account for a 2.6 Msun neutron star, (b) satisfy the original constraint on the tidal deformability of a 1.4 Msun neutron star, and (c) reproduce ground-state properties of finite nuclei. Yet, for the class of models explored in this work, we find that the stiffening of the equation of state required to support super-massive neutron stars is inconsistent with either constraints obtained from energetic heavy-ion collisions or from the low deformability of medium-mass stars. Thus, we speculate that the maximum neutron star mass can not be significantly higher than the existing observational limit and that the 2.6 Msun compact object is likely to be the lightest black hole ever discovered.