Hybrid star models in the light of new multi-messenger data

Recent astrophysical mass inferences of compact stars (CSs) HESS J1731-347 and PSR J0952-0607, with extremely small and large masses respectively, as well as the measurement of the neutron skin of Ca in the CREX experiment challenge and constrain the models of dense matter. In this work, we examine the concept of hybrid stars - objects containing quark cores surrounded by nucleonic envelopes - as models that account for these new data along with other (multimessenger) inferences. We employ a family of 81 nucleonic equations of state (EoSs) based on covariant density functional with variable skewness and slope of symmetry energy at saturation density and a constant speed-of-sound EoS for quark matter. For each nucleonic EoS, a family of hybrid star EoS is generated by varying the transition density from nucleonic to quark matter, the density jump at the transition, and the speed-of-sound. These models are tested against the data from GW170817 and J1731-347, which favor low-density soft EoS and PSR J0592-0607 and J0740+6620, which require high-density stiff EoS. We then examine the occurrence of twin configurations and quantify the ranges of masses and radii that they can possess. It is shown that including J1731-347 data favors EoS models which predict low-mass twin stars with masses M ≲ 1.3 M_⊙ that can be realized if the deconfinement transition density is low. If combined with a large speed-of-sound in quark matter such models allow for maximum masses of hybrid stars in the range 2.0-2.6 M_⊙.