Are NICER and GW170817 Constraints Suggesting a Compactified Scenario for Neutron Stars?

Abstract Astrophysical observations from Neutron Star Interior Composition Explorer and gravitational waves constrain the properties of matter at the cores of neutron stars (NSs), enabling us to probe high-density matter with greater accuracy. To understand its implications for NSs, three distinct class-agnostic equation-of-state (EoS) ensembles are constructed using the speed-of-sound parameterization. Bayesian analysis is employed to constrain the parameters, namely, the squared speed of sound and chemical potential using the observational data. The observations effectively constrain the low-density region of the EoS. Astrophysical observations favour a softer EoS at low density, with the phase transition occurring at intermediate densities. If at all there is a discontinuous EoS, the discontinuity is preferably small. The EoS that yields the maximum NS mass shows considerable stiffening at very low density, providing pressure support to achieve the maximum mass. In contrast, the EoS that generates maximum-compactness stellar structure has a softer low-density region, followed by pronounced stiffening, yielding the maximum-compactness configuration. The observationally favoured EoS shares the same qualitative structure as the maximum-compactness EoS: relative softness at intermediate densities transitioning to a stiff region at high densities, a configuration gravity naturally favors.