Probing fermionic asymmetric dark matter cores using global neutron star properties

It is possible for asymmetric dark matter (ADM) to accumulate in neutron star interiors and affect their global properties. Considering the effects of this accumulation, neutron star mass-radius measurements can deliver new insights into the cold dense matter equation of state (EoS). In this paper, we employ Bayesian parameter estimation using real and synthetic neutron star mass-radius data to infer constraints on the combined baryonic matter and fermionic ADM EoS, where the fermionic ADM forms a core in the neutron star interior. Using currently available mass-radius data, we find that the lower bound of the ratio between ADM effective self-repulsion strength ($g_\chi/m_\phi$) and particle mass ($m_\chi$) can be constrained at the 68\% (95\%) credible level to $10^{-6.59}$ ($10^{-7.77}$). We also find that, if neutron star mass-radius measurement uncertainties are reduced to the 2\% level, the constraints on the lower bound of the ratio of $g_\chi/m_\phi$ to $m_\chi$ can be improved to $10^{-6.49}$ and $10^{-7.68}$ at the 68\% and 95\% credible levels, respectively. However, all other combinations, of $m_\chi$, $g_\chi$, and the ADM mass-fraction, $F_\chi$, (i.e., the ratio of the gravitational ADM mass to the gravitational mass of the neutron star) are unconstrained. Furthermore, in the pressure-energy density and mass-radius planes, the inferences which include the possibility of fermionic ADM cores are nearly identical with the inferences that neglect fermionic ADM for $F_\chi \leq 1.7\%$ and neutron star mass-radius uncertainties $\geq 2\%$. Therefore, we find that neutron star mass-radius measurements can constrain the ratio of $g_\chi/m_\phi$ to $m_\chi$ and that neutron stars with ADM are indistinguishable from purely baryonic stars. This implies that neutron stars with ADM are equally as consistent with the available mass-radius data as neutron stars without ADM.