Impact of the PSR J 0740 +6620 radius constraint on the properties of high-density matter

X-ray pulse profile modeling of PSR J 0740 +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_⊙ . 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 0.7 5_-0.24^+0.25 c² 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 J 0740 +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 β -equilibrium.