Neutron stars more compact than black holes in quasi-topological gravity: Equilibrium configurations and radial stability

TL;DR

This paper investigates neutron star models in quasi-topological gravity, demonstrating they can be more compact than black holes, and analyzes their equilibrium and stability properties across various conditions.

Contribution

It provides a detailed analysis of ultra-compact neutron stars in quasi-topological gravity, showing their stability and exceeding black-hole compactness limits.

Findings

Neutron stars in QTG can surpass black-hole compactness limits.

QTG corrections become more significant at high densities.

Some configurations stabilized by QTG are radially unstable in GR.

Abstract

Within general relativity, black holes are widely regarded as the ultimate benchmark for compactness in the Universe. Recently, however, neutron star models have been constructed in a higher-curvature theory -- quasi-topological gravity (QTG) -- whose compactness can exceed the black-hole limit~\cite{LD19666}. Here we present a detailed analysis of both the equilibrium structure and radial stability of such configurations in QTG. By examining several representative equations of state and different values of the gravitational coupling constant, we find that in the high-central-density regime the compactness exceeding the black-hole bound exhibits a universal behavior in QTG. We further show that QTG corrections grow increasingly significant at large central densities and can stabilize configurations that are radially unstable in general relativity over a broad parameter range. These results establish ultra-compact neutron stars in QTG as theoretically viable strong-field configurations and provide a foundation for further investigations of their dynamical and phenomenological implications.