Relativistic hybrid stars in light of the NICER PSR <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">J</mml:mi><mml:mn>0740</mml:mn><mml:mo>+</mml:mo><mml:mn>6620</mml:mn></mml:mrow></mml:math> radius measurement
Jia Jie Li, Armen Sedrakian, Mark Alford
Abstract
We explore the implications of the recent radius determination of PSR $\mathrm{J}0740+6620$ by the NICER experiment, combined with the neutron skin measurement by the PREX-II experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. We combine a covariant density-functional nucleonic equation of state (EOS) with a constant-speed-of-sound EOS for quark matter. We show that the radius and tidal deformability ranges obtained from GW170817 can be reconciled with the implication of the PREX-II experiment if there is a phase transition to quark matter in the low-mass compact star. In the high-mass segment, the EOS needs to be stiff to comply with the large-radius inference for PSR $\mathrm{J}0740+6620$ and $\mathrm{J}0030+0451$ with masses $M\ensuremath{\simeq}2\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and $M\ensuremath{\simeq}1.4\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$. We show that twin stars are not excluded, but the mass and radius ranges (with $M\ensuremath{\ge}{M}_{\ensuremath{\bigodot}}$) are restricted to narrow domains $\mathrm{\ensuremath{\Delta}}{M}_{\mathrm{twin}}\ensuremath{\lesssim}0.05\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{twin}}\ensuremath{\sim}1.0\text{ }\text{ }\mathrm{km}$. We also show that the existence of twin configurations is compatible with the light companion in the GW190814 event being a hybrid star in the case of values of the sound-speed square $s=0.6$ and $s=1/3$.