QHC21 equation of state of neutron star matter -- in light of 2021 NICER data

The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation in neutron star radii from $1.4M_\odot$ to $2.1M_\odot$, suggest that the neutron star equation of state remains relatively stiff up to baryon densities $n \sim$ 2-4 times nuclear saturation density, $n_0$ -- the region where we expect hadronic matter to be undergoing transformation into quark matter. To delineate the physics from the nuclear to the quark matter regimes we use the quark-hadron-crossover (QHC) template to construct an updated equation of state, QHC21. We include nuclear matter results primarily based on chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces, thus covering the range of uncertainties in the nuclear equation of state. To allow for a possible early transition to quark degrees of freedom we begin the crossover regime from nucleons to quarks at $1.5n_0$. The resulting equations of state are stiffer than our earlier QHC19 at $\lesssim 2n_0$, predicting larger radii in substantial agreement with the NICER data, with accompanying peaks in sound velocity at 2-4$n_0$. We discuss possible microscopic mechanisms underlying stiffening of the equation of state.