Implications of Fermionic Dark Matter Interactions on Anisotropic Neutron Stars

Abstract The presence of dark matter (DM) within a neutron star (NS) can substantially influence the macroscopic properties. It is commonly assumed that the pressure inside an NS is isotropic, but in reality, pressure is locally anisotropic. This study explores the properties of anisotropic NS with a subfraction of DM (isotropic) trapped inside. Implementing a two-fluid formalism with three distinct classes of equation of state: AP3 (a realistic nucleon–nucleon interaction model), BSk22 (modeling atomic nuclei and neutron matter), and MPA1 (considering relativistic effects in nuclear interactions). The properties of NS, such as mass ( M ), radius ( R ), and dimensionless tidal deformability (Λ), for various DM-anisotropic configurations, have been rigorously tested against observational constraints. These constraints include data from the binary NS merger GW170817, NICER X-ray measurements, and pulsar mass–radius observations. We observe that with increasing DM subfraction, higher anisotropies could also satisfy the observational constraints for Yukawa coupling strength g = 10 −5 , which satisfies the DM self-interaction cross-section ( σ ) i.e 0.1–10 cm −2 g −1 . Furthermore, increasing the coupling ( g ) between DM and its mediator leads to the formation of a core–halo structure, with a DM halo surrounding the baryonic matter. Specifically, for coupling values of g = 10 −4 , 10 −3.7 , and 10 −3.5 , we observe that the maximum radius ( R max ) decreases with increasing anisotropy, which contrasts with the behavior at g = 10 −5 and in scenarios with no DM. Our analysis indicates that binary pulsar systems could potentially constrain the extent of admixed anisotropic NS or, more optimistically, provide evidence for the existence of DM-admixed anisotropic NS.