Linear analysis of I-C-Love universal relations for neutron stars

TL;DR

This paper presents a linear analysis of the I-C and I-Love universal relations in neutron stars, revealing that their universality stems from background star structure rather than EOS differences, offering a new perspective on these relations.

Contribution

It introduces a linear framework to analyze the origin of neutron star universal relations, separating EOS effects from background structure influences.

Findings

The deviation in universal relations can be factorized into EOS-dependent and structure-dependent parts.

The structure-dependent factor remains small across realistic EOSs, explaining universality.

The linear approximation is valid for analyzing differences among realistic EOSs.

Abstract

Neutron stars (NSs) are excellent laboratories for testing gravity theories as they are strongly self-gravitating bodies and have rich observational phenomena. However, strong-field gravity effects in NS could be degenerate with their equation of state (EOS) which is largely unknown. Fortunately, there exist the so-called universal relations among the NS macroscopic quantities that are found to be insensitive to the underlying EOS. Studying the origin of these relations can lead to a better understanding of NSs and the gravitational interaction. We develop a new perspective of view to analyze the I-C and I-Love universal relations for NSs. At the linear order, we separate the deviation of the universal relations into two factors that are multiplied together. One is the EOS difference while the other factor only depends on the background star structure. The smallness of the second factor that does not depend on the EOS difference then indicates the origin of the universality. We discuss the validity of our linear approximation when considering the difference among realistic EOSs. Our study can be regarded as a new frame for quantitative representation of the universality and may provide new insights to the universal relations of NSs.