Implications of NICER for neutron star matter: the QHC21 equation of state

TL;DR

This paper develops the QHC21 equation of state for neutron star matter, integrating nuclear and quark matter physics, to explain NICER observations of neutron star radii and internal structure.

Contribution

The paper introduces the QHC21 equation of state, incorporating recent NICER data and nuclear physics, with a focus on the hadron-quark transition in neutron stars.

Findings

QHC21 predicts larger neutron star radii consistent with NICER data.

The equation features a peak in sound velocity around 2-4 times nuclear saturation density.

QHC21 is stiffer than previous models below 2 times nuclear saturation density.

Abstract

The recent NICER measurement of the radius of the neutron star PSR J0740+6620, and the inferred small variation of radii from 1.4$M_\odot$ to 2.1$M_\odot$, reveal key features of the equation of state of neutron star matter. The pressure rises rapidly in the regime of baryon density $n \sim$ 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 (QHC). 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 $n \sim 1.5n_0$, 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 $\sim 2$-$4 n_0$, is stiffer than the earlier QHC19 below 2$n_0$, predicting larger radii in substantial agreement with the NICER data.