Precisely constraining the properties of neutron stars using new universal relations and astronomical observations

Given the significant uncertainty in the equation of state (EOS) of high-density nuclear matter, establishing EOS-independent universal relations between global properties of neutron stars provides a practical method to constraining unobservable or difficult-to-observe properties using astronomical observations. Constructing universal relations between EOS-dependent properties (e.g., moment of inertia, tidal deformation, etc.) or combined properties (e.g., compactness) is common, and improving the precision of the universal relations may provide stricter constraint on the properties of neutron star. We find that in 3-dimensional space with mass and radius as the base coordinates, the points corresponding to properties of a neutron star described by different EOSs are almost located on the same surface. Thus, the universal relation between the property and the stellar mass-radius can be expressed through describing the surface. We demonstrate that the resulting universal relations have greater precision. For example, we construct high-precision universal relations for the moment of inertia, the f-mode frequency, and the dimensionless tidal deformation with respect to the mass-radius. With the increase in the size and accuracy of NICER observational data for neutron star masses and radii, these universal relations allow for more precise constraints on the unobservable or difficult-to-observe properties.