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

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-circle dot. 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(-0.24)(+0.25) c(2) at 3.60(-1.89)(+2.25) 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.

Automated summary

The X-ray pulse profile modeling of PSR J0740+6620 using data from NICER and XMM-Newton observatories has led to a measurement of its radius, providing insight into the neutron star equation of state (EoS). The analysis mildly disfavors EoSs supporting a disconnected hybrid star branch and reveals core matter with strong interactions violating the conformal sound-speed bound. Results suggest a relatively stiff EoS and limit the density range probed by observations of cold, non-rotating neutron stars in beta-equilibrium.