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

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.

Figures (2)

• Figure 1: Radius excess $\Delta R\equiv R-R_{\rm bm}$ (top) and fractional mass excess $\Delta M/M\equiv (M-M_{\rm bm})/M$ (bottom) as functions of the gravitational mass $M$ for dark–matter–admixed NSs with an interacting dark sector. Here $R$ and $M$ are the total (gravitational) radius and mass that source the exterior spacetime, while $R_{\rm bm}$ and $M_{\rm bm}$ are the baryonic (electromagnetic) counterparts inferred from the luminous component. Curves compare fermionic dark–matter masses $m_\chi=1~\mathrm{GeV}$ and $400~\mathrm{MeV}$, two central baryon fractions $y_{\rm bm}=\{0.982,\,0.965\}$, and internal dark–sector partitions $x_\chi=\{1.0,\,0.97\}$ (the latter corresponding to a 3% vacuum–like fraction when $x_\chi=0.97$). Lighter $m_\chi$ and smaller $y_{\rm bm}$ produce extended halos that substantially increase $R$, while the associated change in $M$ is present but comparatively modest; introducing a small vacuum–like admixture ($x_\chi<1$) leads to a marked reduction in the halo’s radial extent and mass.
• Figure 2: Mass–radius relations of NSs modified by dark sector interactions, constrained by observations (background clouds). Two primary markers are presented: pink curves, representing the contribution from the luminous radius alone, and blue curves, indicating the total gravitational radius of the objects. The solid gray curve denotes the baseline case in which only baryonic matter is considered ($y_m = 1.0$).