Realistic Anisotropic Neutron Stars: Pressure Effects

TL;DR

This paper investigates how anisotropic pressure influences neutron star properties, showing that anisotropy can increase maximum mass and explain observations like GW190814, with a new universal relation for binding energy.

Contribution

It introduces a model for anisotropic neutron stars with a quasi-local anisotropy measure and derives a universal binding energy relation, expanding understanding of neutron star structure.

Findings

Anisotropy can increase maximum neutron star mass by up to 15%.

Neutron stars can exceed 2.5 solar masses due to anisotropy.

A universal binding energy relation as a function of compactness and anisotropy is proposed.

Abstract

In this paper, we study the impact of anisotropy on neutron stars with different equations of state, which have been modeled by a piecewise polytropic function with continuous sound speed. Anisotropic pressure in neutron stars is often attributed to interior magnetic fields, rotation, and the presence of exotic matter or condensates. We quantify the presence of anisotropy within the star by assuming a quasi-local relationship. We find that the radial and tangential sound velocities constrain the range of anisotropy allowed within the star. As expected, the anisotropy affects the macroscopic properties of stars, and it can be introduced to reconcile them with astrophysical observations. For instance, the maximum mass of anisotropic neutron stars can be increased by up to 15\% compared to the maximum mass of the corresponding isotropic configuration. This allows neutron stars to reach masses greater than $2.5M_\odot$, which may explain the secondary compact object of the GW190814 event. Additionally, we propose a universal relation for the binding energy of an anisotropic neutron star as a function of the star's compactness and the degree of anisotropy.