Anisotropic Neutron Stars Modelling: Constraints in Krori-Barua Spacetime

TL;DR

This paper models anisotropic neutron stars using Krori-Barua spacetime without assuming an equation of state, deriving constraints on compactness and boundary density that align with observational data, and estimating a maximum mass of 4.1 solar masses.

Contribution

It introduces a novel modeling approach for anisotropic neutron stars in Krori-Barua spacetime without presupposing an equation of state, providing new constraints and observational consistency.

Findings

Maximum allowed compactness is 0.71.

Boundary density matches nuclear saturation density.

Maximum neutron star mass estimated at 4.1 solar masses.

Abstract

Dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condesation. Therefore an anisotropic neutron star core seems more realistic than an ideally isotropic one. We model anisotropic neutron stars working in the Krori-Barua (KB) ansatz without preassuming an equation of state. We show that the physics of general KB solutions is encapsulated in the compactness. Imposing physical and stability requirements yields a maximum allowed compactness $2GM/Rc^2 < 0.71$ for a KB-spacetime. We further input observational data from numerous pulsars and calculate the boundary density. We focus especially on data from the LIGO/Virgo collaboration as well as recent independent measurements of mass and radius of miilisecond pulsars with white dwarf companions by the Neutron Star Interior Composition Explorer (NICER). For these data the KB-spacetime gives the same boundary density which surprisingly equals the nuclear saturation density within the data precision. Since this value designates the boundary of a neutron core, the KB-spacetime applies naturally to neutron stars. For this boundary condition we calculate a maximum mass of 4.1 solar masses.