Anisotropic neutron stars modelling: constraints in Krori–Barua spacetime

Abstract

Dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condensation. 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 $$2GM/Rc^2 < 0.71$$ 2 G M / R c 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.

Anisotropic compact star with a linear pressure-density relationship

“…To show that the solution can be used as a viable model for compact observed sources, we consider the pulsar 4U 1608 − 52 whose mass and radius are estimated to be M = 1.57 +0.30 −0.29 M and R = 9.8 +1.8 −1.8 km, respectively 62 . For the given mass and radius, we have determined the values of the constants C = −0.0070827, β = −0.002 and A = 0.00956915 for arbitrarily chosen values of α = 0.19, B = −4.25304528 and δ = 1.2.…”

Analytical model of low mass strange stars in 2+1 spacetime

“…This repulsiveness in anisotropic force enhances the stability of the star resulting in the star to be more compact than the isotropic one. [33][34][35]…”

Anisotropic magnetized neutron star

“…For example, there are no equations of state, and this is often seen as standard for perfect liquids. It is notable that the difference between the principal pressures is known as the anisotropic parameter denoted by [25]. Recently, the assumption of pressure anisotropy for the compact objects has been also discussed by Herrera [26].…”