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

The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation of radii from 1.4 to 2.1 M (circle dot), reveal key features of the equation of state of neutron star matter. The pressure rises rapidly in the regime of baryon density n similar to 2-4 times nuclear saturation density, n (0)-the region where we expect hadronic matter to be undergoing transformation into quark matter-and the pressure in the nuclear regime is greater than predicted by microscopic many-body variational calculations of nuclear matter. To incorporate these insights into the microscopic physics from the nuclear to the quark matter regimes, we construct an equation of state, QHC21, within the framework of quark-hadron crossover. We include nuclear matter results primarily based on the state-of-the-art chiral effective field theory, but also note results of using nuclear matter variational calculations based on empirical nuclear forces. We employ explicit nuclear degrees of freedom only up to n similar to 1.5 n (0), in order to explore the possibility of further physical degrees of freedom than nucleonic here. The resulting QHC21, which has a peak in sound velocity in similar to 2-4 n (0), is stiffer than the earlier QHC19 below 2 n (0), predicting larger radii in substantial agreement with the NICER data.

Automated summary

The recent NICER measurement of the radius of the neutron star PSR J0740+6620 shows that the equation of state of neutron star matter has key features. In the regime of baryon density 2-4 times nuclear saturation density, the pressure rises rapidly, indicating the possible transformation of hadronic matter into quark matter. The resulting equation of state, QHC21, predicts larger radii in substantial agreement with the NICER data.