Impact of NICER's Radius Measurement of PSR J0740+6620 on Nuclear Symmetry Energy at Suprasaturation Densities

TL;DR

This paper uses NICER's recent measurement of PSR J0740+6620's radius to constrain the nuclear symmetry energy at suprasaturation densities, ruling out super-soft symmetry energy models.

Contribution

It demonstrates how neutron star radius measurements can tightly constrain the nuclear symmetry energy at high densities, refining previous bounds.

Findings

Lower radius limit for PSR J0740+6620 sets a tighter boundary on symmetry energy.

Super-soft symmetry energy models are disfavoured by the measurement.

Results improve understanding of dense nuclear matter properties.

Abstract

By directly inverting several neutron star observables in the three-dimensional parameter space for the Equation of State of super-dense neutron-rich nuclear matter, we show that the lower radius limit for PSR J0740+6620 of mass $2.08\pm 0.07~M_{\odot}$ from Neutron Star Interior Composition Explorer (NICER)'s very recent observation sets a much tighter lower boundary than previously known for nuclear symmetry energy in the density range of $(1.0\sim 3.0)$ times the saturation density $\rho_0$ of nuclear matter. The super-soft symmetry energy leading to the formation of proton polarons in this density region of neutron stars is clearly disfavoured by the first radius measurement for the most massive neutron star observed reliably so far.