Bayesian Inferences on the Twin-star Scenario from NICER Pulsars

We investigate dense-matter equations of state (EOSs) within a Bayesian framework, focusing on the constraints from the latest NICER mass–radius measurements of PSR J0614−3329 and the massive pulsar PSR J0740+6620, together with PSR J0030+0451 and PSR J0437−4715. For the hadronic sector, we adopt a meta-modeling EOS constrained by these NICER measurements. The hadronic inference indicates that PSR J0614−3329 favors a somewhat softer EOS than the other two ∼1.4 M ⊙ pulsars, while the ∼2 M ⊙ constraint prevents the EOS from becoming too soft. We then introduce a strong first-order phase transition through a constant-speed-of-sound quark-matter segment. Constraining the transition parameters with PSR J0614−3329 as the primary source and XTE J1814−338 as a secondary constraint, we find a preferred transition density of n t ∼ 3.3 n 0 , a sizable energy-density jump of Δ ϵ − Δ ϵ cr ∼ 800 MeV, and a relatively large posttransition sound speed of c s 2 / c 2 ∼ 0.7 . Including the additional constraint from HESS J1731−347 shifts these values to n t ∼ 2.7–2.8 n 0 , Δ ϵ − Δ ϵ cr ∼ 600–700 MeV and c s 2 / c 2 ∼ 0.85 . Such a phase transition generates a disconnected hybrid branch with radii of about 6–7 km at masses around 1.2–1.4 M ⊙ , and strongly suppresses the dimensionless tidal deformability relative to the purely hadronic branch. This pronounced change in tidal deformability is a characteristic signature of the twin-star mechanism and may provide an important observational tool for identifying phase transitions in neutron star matter in future multimessenger measurements. Our results demonstrate that the NICER data, particularly the recently measured PSR J0614−3329, when supplemented by XTE J1814−338, already provide meaningful constraints on both the strength of a first-order phase transition and the stiffness of the posttransition phase in dense matter.