Properties of first-order hadron-quark phase transition from inverting neutron star observables

By inverting the observational data of several neutron star observables in the three-dimensional parameter space of the constant speed of sound (CSS) model while fixing all hadronic equation-of-state parameters at their currently known most probable values, we constrain the three parameters of the CSS model and their correlations. Using two lower radius limits of R_2.01=11.41 km and R_2.01=12.2 km for PSR J0740+6620 obtained from two independent analyses using different approaches by the Neutron Star Interior Composition Explorer (NICER) Collaboration, the speed of sound squared c_QM² in quark matter is found to have a lower limit of 0.35 or 0.43 in units of c², above its conformal limit of c_QM²<1 /3 . An approximately linear correlation between the first-order hadron-quark transition density ρ_t and its strength Δ ɛ is found. Moreover, the presence of twin stars is deemed improbable by the present work.