2025ApJ...991..158P

We present a reanalysis of Neutron Star Interior Composition Explorer observations of PSR J0740+6620 and PSR J0030+0451 in order to test the consistency of various nuclear equations of state (EoSs) within the framework of hybrid star models. In particular, we examine how different surface temperature models for PSR J0030+0451, categorized as scenarios A, B, and C, lead to significantly different mass-radius estimates. We present a comprehensive study constraining the parameters of the constant speed of sound (CSS) model based on representative observational categories. Our findings indicate that for certain hadronic EoSs (for representative cases, we chose one density-independent and one density-dependent), the results remain consistent for lower values of the energy density discontinuity, but discrepancies emerge as the discontinuity increases. Scenarios involving large jumps in energy density are generally disfavored by the requirement of supporting massive neutron stars, while higher values of the speed of sound in the quark matter phase tend to yield better agreement with various observational trends. These results underscore the importance of phase transition characteristics in aligning hybrid star models with current astrophysical observations. We have also constrained the CSS parameters using observational data from PSR J0740+6620 and PSR J0952-0607 by computing the maximum mass supported by these parameters.