Limiting masses and radii of neutron stars and their implications

We combine the equation of state of dense matter up to twice nuclear saturation density n_sat obtained using chiral effective field theory (χ EFT ) and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-by-order calculations up to next-to-next-to-next-to-leading order in the χ EFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities and provide stringent limits on the maximum and minimum radii of ∼1.4 M_⊙ and ∼2.0 M_⊙ stars. Including χ EFT predictions from n_sat to 2 n_sat reduces the permitted ranges of the radius of a 1.4 M_⊙ star, R_1.4, by ∼3.5 km . If observations indicate R_1.4<11.2 km , then our study implies that either the squared speed of sound c_s²>1 /2 for densities above 2 n_sat or that χ EFT breaks down below 2 n_sat . We also comment on the nature of the secondary compact object in GW190814 with mass ≃2.6 M_⊙ and discuss the implications of massive neutron stars >2.1 M_⊙(2.6 M_⊙) in future radio and gravitational-wave searches. Some form of strongly interacting matter with c_s²>0.35 (0.55 ) must be realized in the cores of such massive neutron stars. In the absence of phase transitions below 2 n_sat , the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by χ EFT for the baryon density n_B in the range 1 -2 n_sat . Together they imply that the rapid stiffening required to support a high maximum mass should occur only when n_B≳1.5 -1.8 n_sat .