Are NICER and GW170817 constraints suggesting a compactified scenario for Neutron stars?

TL;DR

This study uses NICER and gravitational wave data to constrain neutron star equations of state, suggesting a soft low-density phase with a transition to stiffness at higher densities, consistent with a compactified scenario.

Contribution

It introduces a Bayesian analysis of three class-agnostic equations of state using speed-of-sound parametrisation, revealing implications for neutron star structure and phase transitions.

Findings

Observations constrain the low-density equation of state.

Phase transition occurs at intermediate densities.

Maximum mass and compactness configurations differ in low-density stiffness.

Abstract

Astrophysical observations from NICER and gravitational wave data constrain the properties of matter at the cores of neutron stars, enabling us to probe high-density matter with greater accuracy. To understand its implications for neutron stars, three distinct class-agnostic equation-of-state ensembles are constructed using the speed-of-sound parametrisation, which can describe matter in neutron-star cores. Bayesian analysis is employed to constrain the parameters, namely, the squared speed of sound and chemical potential, using the observational data. The Bayesian inference shows that the observations effectively constrain the low-density region of the equation of state. The astrophysical bound favours a softer, low-density equation of state in which the phase transition occurs at intermediate densities, thereby reducing the upper mass bounds for neutron stars. For the equation of state with density discontinuity, the discontinuities are preferably small. The equation of state with maximum mass configuration shows considerable stiffening from very low density, providing pressure support to generate maximum mass. In contrast, the equation of state with the maximum compact stellar configuration has a softer low-density equation of state, followed by pronounced stiffening, yielding the maximum compact configuration. The observationally favoured EoS shares the same qualitative structure as the maximum-compactness EoS: relative softness at intermediate densities transitioning to stiffness at high densities, a configuration gravity naturally favours.