Implications of NICER for Neutron Star Matter: The QHC21 Equation of State

The recent NICER measurement of the radius neutron star PSR J0740+6620, and inferred small variation radii from 1.4$M_\odot$ to 2.1$M_\odot$, reveal key features equation state matter. pressure rises rapidly in regime baryon density $n \sim$ 2-4 times nuclear saturation density, $n_0$ -- region where we expect hadronic matter be undergoing transformation into quark is greater than predicted by microscopic many-body variational calculations To incorporate these insights physics regimes, construct an state, QHC21, within framework quark-hadron crossover (QHC). We include results primarily based on state-of-the-art chiral effective field theory, but also note using empirical forces. employ explicit degrees freedom only up \sim 1.5n_0$, order explore possibility further physical nucleonic here. resulting which has a peak sound velocity $\sim 2$-$4 n_0$, stiffer earlier QHC19 below 2$n_0$, predicting larger substantial agreement with data.