Probing fermionic asymmetric dark matter cores using global neutron star properties (2410.00140v2)

Abstract: 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.