2019ApJ...879...99Z

We study the implications of the very recently reported mass M=2.17_–0.10^+0.11M_☉ of PSR J0740+6620 on the equation of state (EOS) of super-dense neutron-rich nuclear matter with respect to existing constraints on the EOS based on the mass M = 2.01 ± 0.04 M_☉ of PSR J0348+0432, the maximum tidal deformability of GW170817, and earlier results of various terrestrial nuclear laboratory experiments. The lower limit of the skewness J₀ measuring the stiffness of super-dense isospin-symmetric nuclear matter is raised from about -220 MeV to -150 MeV, significantly reducing its current uncertainty range. The lower bound of the high-density symmetry energy also increases appreciably leading to a rise of the minimum proton fraction in neutron stars at β-equilibrium from about 0% to 5% around three times the saturation density of nuclear matter. The difficulties for some of the most widely used and previously well tested model EOSs to simultaneously predict both a maximum mass higher than 2.17 M_☉ and a pressure consistent with that extracted from GW170817 present some interesting new challenges for nuclear theories.