The Impact of Anisotropy on Neutron Star Properties: Insights from I-f-C Universal Relations

Abstract

This study presents a universal relation for anisotropic neutron stars, called the $I-f-C$ relation, which accounts for the local anisotropic pressure using the Quasi-Local (QL) Model proposed by Horvat et al. \cite{QL_Model} to describe the anisotropy inside the neutron star. This study analyzes approximately 60 unified tabulated EoS-ensembles, spanning from relativistic to non-relativistic mean-field models, that comply with multimessenger constraints and cover a broad range of stiffness. The results indicate that the relationship between the parameters becomes more robust with positive anisotropy, while it weakens with negative anisotropy. With the help of the GW170817 \& GW190814 tidal deformability limit, a theoretical limit for the canonical $f$-mode frequency for both isotropic and anisotropic stars is established. For isotropic case the canonical $f$-mode frequency for event GW170817 \& GW190814 is $f_{1.4} = 2.605^{+0.487} _ {-0.459}\ \mathrm{kHz}$ and $ f_{1.4} = 2.093^{+0.150} _ {-0.125} \ \mathrm{kHz}$ respectively. These established relationships have the potential to serve as a reliable tool to limit the equation of state of nuclear matter when measurements of relevant observables are obtained.