Shedding Light on the EOS-Gravity Degeneracy and Constraining the Nuclear Symmetry Energy from the Gravitational Binding Energy of Neutron Stars

Xiao-Tao He,Bao-An Li,F J Fattoyev,W G Newton

doi:10.1051/epjconf/201610907002

Abstract

A thorough understanding of properties of neutron stars requires both a reliable knowledge of the equation of state (EOS) of super-dense nuclear matter and the strong-field gravity theories simultaneously. To provide information that may help break this EOS-gravity degeneracy, we investigate effects of nuclear symmetry energy on the gravitational binding energy of neutron stars within GR and the scalar-tensor subset of alternative gravity models. We focus on effects of the slope $L$ of nuclear symmetry energy at saturation density and the high-density behavior of nuclear symmetry energy. We find that the variation of either the density slope $L$ or the high-density behavior of nuclear symmetry energy leads to large changes in the binding energy of neutron stars. The difference in predictions using the GR and the scalar-tensor theory appears only for massive neutron stars, and even then is significantly smaller than the difference resulting from variations in the symmetry energy.

Highlights

IntroductionThere is a possible degeneracy between the models of the neutron-star matter equation of state (EOS) and models of gravity applied to describe their properties
With the goal of providing information that may help break this degeneracy we have studied effects of the nuclear symmetry energy within its current uncertain range on the mass-versus-radius relation and the binding energy of Neutron stars (NSs) within both the GR and the scalar-tensor theory of gravity
We have found that radii of neutron stars are primarily sensitive to the underlying equation of state (EOS) through the density dependence of the symmetry energy

Summary

Introduction

There is a possible degeneracy between the models of the neutron-star matter EOSs and models of gravity applied to describe their properties How to break this degeneracy is a longstanding problem to which many recent studies have been devoted (see e.g., [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]). We find that the variation of either the density slope L or the high-density behavior of nuclear symmetry energy within their uncertainty ranges lead to significant changes in the binding energy of NSs. In particular, the variations are significantly greater than those that result from ST theories of gravity, leading to the conclusion that within those subset of gravity models, measurements of neutron star properties constrain mainly the EOS. Further investigations demonstrate that only EOSs with the soft symmetry energy at high-density are consistent with constraints on the gravitational binding energy of PSR J0737-3039B

The Equation of State of Nuclear Matter

Summary