2026ApJS..282...33C

This study analyzes and contrasts different phenomenological methods used to model the nuclear equation of state (EOS) for neutron star matter based on covariant energy density functionals (CEDFs). Using two complementary methodologies, we seek to capture a comprehensive picture of the potential behaviors of ultradense nucleonic matter and identify the most plausible models based on current observational and experimental constraints. Observational data from radio pulsar timing, gravitational-wave detection of GW170817, and X-ray timing provide critical benchmarks for testing the models. We have derived the EOS posteriors for various CEDF models within the CompactObject package, utilizing recent observational data on neutron stars, state-of-the-art theoretical constraints from calculations in chiral effective field theory for pure neutron matter at low densities, and pQCD-derived constraints. Our analysis has demonstrated that while all considered CEDF models broadly reproduce current astrophysical and theoretical constraints, subtle yet important differences persist among them, with each framework exhibiting distinct characteristics at supranuclear density. This is particularly true for the proton fraction inside neutron stars, but is also supported by the models' behavior with respect to the pure neutron matter EOS and the density dependence of the speed of sound. Our study highlights the sensitivity of predictions for dense matter to the underlying EOS parameterizations and the priors considered.