Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter

TL;DR

This study uses recent neutron star radius measurements to constrain the dense matter equation of state, revealing strong interactions and challenging certain phase transition models within neutron star cores.

Contribution

It introduces a nonparametric Gaussian process model to analyze neutron star data, constraining phase transitions and the sound speed within neutron star cores.

Findings

Disfavors hybrid star branches supporting strong phase transitions.

Violates the conformal sound-speed bound inside neutron star cores.

Limits the central density of PSR J0740+6620 to about 3.6 times nuclear saturation density.

Abstract

X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, with data from the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement's implications for the neutron star equation of state (EoS), employing a nonparametric EoS model based on Gaussian processes and combining information from other x-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors EoSs that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of $6.9$. For such EoSs, the transition mass from the hadronic to the hybrid branch is constrained to lie outside ($1,2$) $M_{\odot}$. We also find that the conformal sound-speed bound is violated inside neutron star cores, which implies that the core matter is strongly interacting. The squared sound speed reaches a maximum of $0.75^{+0.25}_{-0.24}\, c^2$ at $3.60^{+2.25}_{-1.89}$ times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620's central density is only $3.57^{+1.3}_{-1.3}$ times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in $\beta$-equilibrium.