Sub-GeV dark matter in neutron stars: halo morphologies and their suppression by vacuum-like pressure

TL;DR

This paper models neutron stars with a dark sector, showing how vacuum-like pressure suppresses dark matter halos, with implications for gravitational wave and X-ray observations.

Contribution

It introduces a covariant two-fluid framework to analyze how vacuum-like dark energy suppresses dark matter halos in neutron stars.

Findings

Lighter fermionic dark matter supports larger halos with increased radii.

Near 1 GeV mass, halos shrink and radii become closer to baryonic radii.

Vacuum-like admixture reduces halo size and mass differences significantly.

Abstract

We investigate neutron stars that contain a unified dark sector composed of cold, degenerate fermionic dark matter and a vacuum-like dark-energy component. Within a general-relativistic two-fluid framework that allows a covariantly conserved, gradient-driven energy exchange between baryons and the dark sector, we quantify how dark microphysics reshapes global structure when the total gravitational radius need not coincide with the luminous baryonic radius. Using a state-of-the-art baryonic equation of state, we explore the halo-forming mass range for fermionic dark matter with particle masses of 400 MeV and 1 GeV, and we characterize sequences by the difference between the total and luminous radii and by the fractional difference between the total and baryonic masses. We confirm established trends: lighter fermions typically support low-density halos that increase the total radius by several kilometers at nearly fixed mass, whereas masses near 1 GeV tend to shrink halos and make the two radii appreciably closer. Our central new result is that a percent-level vacuum-like admixture markedly reduces halo formation, shrinking the radius difference from several kilometers to sub-kilometer scales and the fractional mass difference to $\lesssim 1\%$. Combined gravitational-wave and X-ray observations offer a practical route to bound the halo size and the allowed vacuum-like fraction.