Topological Shell Structures in Neutron Stars: Effects on Equilibrium, Oscillations, and Gravitational-Wave Signatures

TL;DR

This paper investigates how internal topological shells within neutron stars affect their structure, oscillation modes, and gravitational wave signals, potentially leading to observable signatures in gravitational wave detectors.

Contribution

It introduces a model with a distributional density profile inside neutron stars and analyzes its impact on equilibrium, stability, and gravitational wave emissions.

Findings

Topological shells cause non-monotonic changes in oscillation mode frequencies.

Shells can produce observable effects in gravitational wave signals.

Predicted gravitational wave signatures may be detectable by current and future observatories.

Abstract

We study the structural and dynamical consequences of introducing a distributional density profile inside a neutron star, representing a massless, topological shell located at an arbitrary radius. We incorporate this effect into the structure of neutron star and construct equilibrium sequence for several realistic equations of state. Radial stability is examined through the Sturm-Liouville formulation of the $\ell=0$ perturbation equation, supplemented with a jump condition and imprinting distinct features on the fundamental $f$-mode spectrum. We find strong, non-monotonic variations in the mode frequency relative to standard no-shell models. Using first-principles scaling relations, we estimate various gravitational wave observables such as the damping time, quality factor, luminosity and characteristic strain. These observables are then compared with the sensitivity of Advanced LIGO, and third-generation detectors such as the Einstein Telescope and Cosmic Explorer. Our results demonstrate that internal topological shells can leave potentially observable signatures in the oscillation and gravitational wave properties of neutron stars.