Multi-Messenger and Cosmological Constraints on Dark Matter through Two-Fluid Neutron Star Modeling

TL;DR

This paper develops a two-fluid framework to study dark matter admixed neutron stars, treating NM and DM as gravitationally coupled but otherwise independent components. By employing RMF nuclear matter EOSs (QMC-RMF4, BigApple, NL3) and an asymmetric self-interacting fermionic DM model with a light vector mediator, it analyzes how DM forms core- or halo-dominated structures and how these configurations affect mass-radius relations and tidal deformabilities. Combining GW170817, NICER, PSR J0740+6620 data, and Bullet Cluster constraints, the authors map the allowed DM parameter space; halo-dominated configurations are largely ruled out by tidal deformability, while core-dominated configurations remain viable within NM EOS uncertainties. The results illuminate how small-scale NS observations and large-scale cosmological data jointly constrain DM self-interactions and mediator properties, with implications for DM physics and compact star evolution.

Abstract

In this study, we investigate the impact of dark matter (DM) on neutron stars (NSs) using a two-fluid formalism that treats nuclear matter (NM) and DM as gravitationally coupled components. Employing NM equations of state spanning a wide range of stiffness and a self-interacting asymmetric fermionic DM framework, we explore the emergence of DM core- and halo-dominated structures and their observational implications. Constraints from gravitational waves (GW170817), NICER X-ray measurements (PSR J0030+0451), and pulsar mass limits (PSR J0740+6620) delineate a consistent parameter space for DM properties derived from these multi-messenger observations. DM halo-dominated configurations, while consistent with PSR J0740+6620's mass limits and NICER's radius measurements for PSR J0030+0451, are ruled out by the tidal deformability bounds inferred from the GW170817 event. Consequently, the combined limits inferred from the observational data of GW170817, PSR J0030+0451, and PSR J0740+6620 support the plausibility of DM core-dominated configurations. Constraints on the DM self-interaction strength from galaxy cluster dynamics further refine the DM parameter space permitted by NS observations. This work bridges multi-messenger astrophysics and cosmology, providing insights into DM interactions and their implications for NS structure, evolution, and observational signatures.

Figures (13)

• Figure 1: Configurations of DM admixed NSs with $R_{\rm{NM}}$ and $R_{\rm{DM}}$ representing the surfaces of nuclear matter and DM, respectively.
• Figure 2: Mass-radius ($M-R$) profile for a DM admixed NS with $m_{\chi} = 5$ GeV and $g_{\chi}/m_{\rm{v}} = 0.055$ MeV$^{-1}$, employing the NL3 EOS for baryonic matter, with the same central energy density for baryonic matter and DM. The blue dashed line represents the baryonic mass $M_{\rm{NM}}$ as a function of its radius $R_{\rm{NM}}$, while the red dashed line shows the DM mass $M_{\rm{DM}}$ as a function of its radius $R_{\rm{DM}}$. The total mass-radius relationship $M(R)$ is shown by markers, differentiated based on whether the baryonic radius is larger ($R_{\rm{NM}}$$>$$R_{\rm{DM}}$, blue open circle) or smaller ($R_{\rm{NM}}$$<$$R_{\rm{DM}}$, red crosses) than the DM radius. Five specific configurations are highlighted at $M=0.6 M_{\odot}$, $0.9 M_{\odot}$, $1.3 M_{\odot}$, $1.9 M_{\odot}$ and $2.2 M_{\odot}$. The total mass $M$ for these configurations are shown as star markers on the $M (R)$ curve, connected by arrows to their corresponding points on the $M_{\rm{NM}}$ ($R_{\rm{NM}}$) and $M_{\rm{DM}}$ ($R_{\rm{DM}}$) curves.
• Figure 3: Total mass-radius ($M-R$) profiles for DM admixed NSs using QMC-RMF4 as the NM EOS, plotted for different values of the DM self-coupling parameter ($g_{\chi}/m_{\rm{v}}$), which are in the unit of MeV$^{-1}$. The black dotted line in each panel represents the $M$-$R$ profile of a pure baryonic NS, calculated using the single-fluid TOV with QMC-RMF4 EOS. Each grid corresponds to a different ratio of DM to NM central energy density: ${\cal E}_{\rm c}^{\rm DM} = {\cal E}_{\rm c}^{\rm NM}$ (left), ${\cal E}_{\rm c}^{\rm DM} = 2 {\cal E}_{\rm c}^{\rm NM}$ (center), and ${\cal E}_{\rm c}^{\rm DM} = 4 {\cal E}_{\rm c}^{\rm NM}$ (right). The color-coded curves represent various values of $g_{\chi}/m_{\rm{v}}$ , ranging from 0.01 to 0.06 MeV$^{-1}$. Observational constraints from PSR J0740+6620 and PSR J0030+0451 are depicted as shaded bands and contours.
• Figure 4: Total mass $M$, radii ($R_{\rm{NM}}$ and $R_{\rm{DM}}$), and mass fractions for admixed NS configurations as functions of NM central energy density (${\cal E}_{\rm c}^{\rm NM}$), constructed using QMC-RMF4 EOS for baryonic matter. The three columns correspond to different central energy density ratios: ${\cal E}_{\rm c}^{\rm DM} = {\cal E}_{\rm c}^{\rm NM}$ (left), ${\cal E}_{\rm c}^{\rm DM} = 4 {\cal E}_{\rm c}^{\rm NM}$ (center), and ${\cal E}_{\rm c}^{\rm DM} = 8{\cal E}_{\rm c}^{\rm NM}$ (right). The color coding in each panel represents varying DM self-coupling strengths ($g_{\chi}/m_{\rm{v}}$) ranging from 0.00 to 0.05 MeV$^{-1}$. The total mass ($M$) is presented in the first row, with observational constraints from PSR J0740+6620 depicted as shaded regions and the maximum mass for each case marked with solid dots. The middle row compares the radii of NM (solid lines) and DM (dotted lines), with observational constraints from PSR J0030+0451 highlighted as shaded regions. The lower row illustrates the mass fractions of NM $\left({\cal F}_{\rm{NM}}=M_{\rm{NM}}/M, \,{\rm{solid\,\,lines}} \right)$ and DM $\left({\cal F}_{\rm{DM}}=M_{\rm{DM}}/M, \,{\rm{dotted\,\,lines}} \right)$.
• Figure 5: Core-halo configuration boundaries for DM admixed NSs, constructed using QMC-RMF4 EOS for NM and varying DM self-coupling strengths ($g_{\chi}/m_{\rm v}$). The plot shows the transition between core-dominated ($R_{\rm NM} > R_{\rm DM}$) and halo-dominated ($R_{\rm NM} < R_{\rm DM}$) configurations as a function of the central energy density ratio ($\mathcal{E}_{\rm c}^{\rm DM}/\mathcal{E}_{\rm c}^{\rm NM}$) and $g_{\chi}/m_{\rm v}$. The blue solid line represents the core-halo boundary corresponding to configurations with the maximum mass, while the green solid line indicates the boundary for $1.4 \,M_{\odot}$ configurations. The hatched region with black dots indicates configurations where the maximum mass is less than $1.4 \,M_{\odot}$.