Neutron Stars and Gravitational Waves: the Key Role of Nuclear Equation of State

TL;DR

This paper reviews how nuclear equations of state influence neutron star properties and gravitational wave observations, providing constraints on dense matter physics through recent astrophysical data and theoretical modeling.

Contribution

It introduces a parametrization of the nuclear equation of state using the speed of sound and applies it to recent gravitational wave events to constrain dense matter properties.

Findings

Constraints on the nuclear equation of state from GW170817 and GW190425

Insights into the maximum mass of neutron stars from GW190814

Implications for phase transitions in dense nuclear matter

Abstract

Neutron stars are the densest known objects in the universe and an ideal laboratory for the strange physics of super-condensed matter. Theoretical studies in connection with recent observational data of isolated neutron stars, as well as binary neutron stars systems, offer an excellent opportunity to provide robust solutions on the dense nuclear problem. In the present work, we review recent studies concerning the applications of various theoretical nuclear models on a few recent observations of binary neutron stars or neutron-star--black-hole systems. In particular, using a simple and well-established model, we parametrize the stiffness of the equation of state with the help of the speed of sound. Moreover, in comparison to the recent observations of two events by LIGO/VIRGO collaboration, GW170817 and GW190425, we suggest possible robust constraints. We also concentrate our theoretical study on the resent observation of a compact object with mass~$\sim 2.59_{-0.09}^{+0.08}~M_{\odot}$ (GW190814 event), as a component of a system where the main companion was a black hole with mass $\sim 23~M_{\odot}$. There is scientific debate concerning the identification of the low mass component, as it falls into the neutron-star--black-hole mass gap. This is an important issue since understanding the nature of GW190814 event will offer rich information concerning the upper limit of the speed of sound in dense matter and the possible phase transition into other degrees of freedom. We systematically study the tidal deformability of a possible high-mass candidate existing as an individual star or as a component in a binary neutron star system. Finally, we provide some applications of equations of state of hot, dense nuclear matter in hot neutron stars, protoneutron stars, and binary neutron star merger remnants.