Astrophysical Constraints on Hadron--Quark Crossover in Hybrid Neutron Stars
We investigate astrophysical constraints on hybrid neutron-star equations of state constructed by combining an extended linear sigma model description of quark matter with several representative hadronic equations of state through a smooth hadron--quark crossover. Four different hadronic models are examined in order to explore the model dependence of the resulting hybrid equations of state. After imposing stability, causality, perturbative QCD matching, tidal deformability, and maximum-mass constraints, only the eLSM+GMSR(BSK16) construction remains fully consistent with current astrophysical observations. The surviving parameterizations are further constrained using Bayesian inference based on NICER mass--radius measurements and the GW170817 gravitational-wave event. The posterior distributions favor a broad hadron--quark crossover occurring at relatively high baryon densities, while the formation of extended pure quark cores is strongly disfavored. Instead, the current observations favor neutron stars containing an extended mixed hadron--quark transition region. We also investigate the implications of the inferred maximum-mass distribution for the secondary object of GW190814 and find that, within the present model and adopted observational constraints, its interpretation as a neutron star is unlikely.