On the minimum radius of very massive neutron stars

TL;DR

This paper explores how measurements of neutron star radii and maximum masses can constrain the dense matter equation of state, especially regarding the presence and nature of quark matter transitions in very massive neutron stars.

Contribution

It analyzes the relationship between neutron star radii and maximum mass, contrasting hadronic and quark matter models to constrain the properties of dense matter.

Findings

Lower bounds on maximum mass exclude soft quark matter.

Upper bounds on maximum mass disfavor low-density stiff quark matter.

Radius measurements can help distinguish different quark matter transition scenarios.

Abstract

Prospects of establishing the radii of massive neutron stars in PSR J1614-2230 and PSR J0740+6620 from NICER and Chandra observatories hold the potential to constrain the equation of state (EoS) of matter to densities well beyond those encountered in canonical stars of mass $\sim 1.4\,{\rm M}_{\odot}$. In this work, we investigate the relation between the radii of very massive neutron stars up to the maximum mass, $M_{\rm max}$, supported by dense matter EoSs. Results from models with hadronic matter are contrasted with those that include a first-order hadron-to-quark phase transition. We find that a lower bound on $M_{\rm max}$ with an upper bound on the radius of massive pulsars serves to rule out too soft quark matter, and an upper bound on $M_{\rm max}$ with a lower bound on the radius of massive pulsars strongly disfavors a transition into too-stiff quark matter appearing at low densities. The complementary role played by radius inferences from future gravitational wave events of inspiraling binary neutron stars is also briefly discussed.