Mass and radius of the most massive neutron star: The probe of the equation of state and perturbative QCD

Recently, an association of GW190425 and FRB 20190425A had been claimed and a highly magnetized neutron star (NS) remnant was speculated. Given the ∼ 2.5-h delay of the occurrence of FRB 20190425A, a uniformly rotating supramassive magnetar is favored since the differential rotation would have been promptly terminated by the magnetic braking. The required maximum gravitational mass (M_ TOV) of the nonrotating NS is ≈ 2.77M_⊙, which is strongly in tension with the relatively low M_ TOV≈ 2.25M_⊙ obtained in current equation of state (EOS) constraints incorporating perturbative quantum chromodynamics (pQCD) information. However, the current mass-radius and mass-tidal deformability measurements of NSs alone do not convincingly exclude the high M_ TOV possibility. By performing EOS constraints with mock measurements, we find that with a 2% determination for the radius of PSR J0740+6620-like NS it is possible to distinguish between the low and high M_ TOV scenarios. We further explore the prospect to resolve the issue of the appropriate density to impose the pQCD constraints with future massive NS observations or determinations of M_ TOV and/or R_ TOV. It turns out that measuring the radius of a PSR J0740+6620-like NS is insufficient to probe the EOSs around 5 nuclear saturation density, where the information from pQCD becomes relevant. The additional precise M_ TOV measurements anyhow could provide insights into the EOS at such a density. Indeed, supposing the central engine of GRB 170817A is a black hole formed via the collapse of a supramassive NS, the resulting M_ TOV≈ 2.2M_⊙ considerably softens the EOS at the center of the most massive NS, which is in favor of imposing the pQCD constraint at density beyond the one achievable in the NSs.