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

Abstract

Recent astrophysical mass inferences of compact stars 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. 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 inferences. We employ a family of 81 nucleonic equations of state (EoSs) 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 EoSs is generated by varying the transition density, the energy jump, and the speed of sound. These models are tested against the data from GW170817 and J1731-347, which favor low-density soft EoS and J0592-0607 and J0740+6620, which require high-density stiff EoS. The addition of J0592-0607's mass measurement to the constraints has no significant impact on the parameter space of the admissible EoS, but allows us to explore the potential effect of pulsars more massive than J0740+6620, if such exists. 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 EoSs that predict low-mass twins with $M \lesssim 1.3\,M_{\odot}$ that can be realized if the deconfinement transition density is low. If combined with large speed of sound in quark matter such models allow for maximum masses of hybrid stars in $2.0$--$2.6\,M_{\odot}$.