Scalar and vector dark matter admixed neutron stars with linear and quadratic couplings

We investigate the effects of dark scalar- and vector-mediated interactions on dark matter admixed neutron stars, employing the two-fluid formalism. We adopt three different nuclear equations of state—BSk22, MPA1 and APR4—to describe the baryonic sector, while the dark component consists of fermionic particles within a relativistic mean field framework. We consider both linear and quadratic scalar interactions with the dark fermion, including a quartic self-interaction in the latter case. The parameters of the dark matter (DM) models are inferred via a Bayesian analysis that incorporates data from NICER observations and binary neutron star merger detections. The neutron star configurations obtained from the selected model parameters develop dark matter cores, leading to more compact objects with smaller masses and radii. Our findings suggest that scalar interactions generally have a weaker impact on the stellar structure compared to vector-mediated ones, though quantitative differences arise. In particular, quadratic scalar couplings suppress the net attractive interaction, allowing for larger dark matter fractions to be accreted. We also compute the sound speed of DM, finding that the scalar and quadratic interactions modify the stiffness of the dark equation of state while respecting causality: vector repulsion enhances the sound speed, whereas scalar attraction tends to soften it. We compare our results with GW1708017, GW190425 and NICER data and constrain DM couplings and mass.