2021ApJ...921..111Z

By directly inverting several neutron star (NS) observables in the three-dimensional parameter space for the equation of state of super-dense neutron-rich nuclear matter, we show that the lower radius limit for PSR J0740+6620 of mass 2.08 ± 0.07 M_☉ from Neutron Star Interior Composition Explorer (NICER)'s very recent observation sets a much tighter lower boundary than previously known for nuclear symmetry energy in the density range of (1.0 ∼ 3.0) times the saturation density ρ₀ of nuclear matter. The super-soft symmetry energy leading to the formation of proton polarons in this density region of NSs is clearly disfavored by the first radius measurement for the most massive NS observed reliably so far.