Inferring the neutron star equation of state with nuclear-physics informed semiparametric models

TL;DR

This paper introduces a semiparametric framework combining nuclear physics and astrophysical data to better constrain the dense matter equation of state in neutron stars, improving understanding of their internal structure and maximum mass.

Contribution

The work develops a novel semiparametric EoS model integrating nuclear physics constraints with Gaussian Processes, extending the exploration of high-density matter and supporting heavy pulsars.

Findings

Maximum neutron star mass above 3.2 solar masses supported

Pressure at twice nuclear saturation density estimated at 1.98e34 dyn/cm^2

Neutron star radius for 1.4 solar masses is approximately 11.4 km

Abstract

Over the past decade, an abundance of information from neutron-star observations, nuclear experiments and theory has transformed our efforts to elucidate the properties of dense matter. However, at high densities relevant to the cores of neutron stars, substantial uncertainty about the dense matter equation of state (EoS) remains. In this work, we present a semiparametric EoS framework aimed at better integrating knowledge across these domains in astrophysical inference. We use a Meta-model at low densities, and Gaussian Process extensions at high densities. Comparisons between our semiparametric framework to fully nonparametric EoS representations show that imposing nuclear theoretical and experimental constraints through the Meta-model up to nuclear saturation density results in constraints on the pressure up to twice nuclear saturation density. We show that our Gaussian Process trained on EoS models with nucleonic, hyperonic, and quark compositions extends the range of EoS explored at high density compared to a piecewise polytropic extension schema, under the requirements of causality of matter and of supporting the existence of heavy pulsars. We find that maximum TOV masses above $3.2 M_{\odot}$ can be supported by causal EoS compatible with nuclear constraints at low densities. We then combine information from existing observations of heavy pulsar masses, gravitational waves from binary neutron star mergers, and X-ray pulse profile modeling of millisecond pulsars within a Bayesian inference scheme using our semiparametric EoS prior. With current astrophysical observations, we find a favored pressure at two times nuclear saturation density of $P(2\rho_{\rm nuc}) = 1.98^{+2.13}_{-1.08}\times10^{34}$ dyn/cm$^{2}$, a radius of a $1.4 M_{\odot}$ neutron star value of $R_{1.4} = 11.4^{+0.98}_{-0.60}$\;km, and $M_{\rm max} = 2.31_{-0.23}^{+0.35} M_{\odot}$ at the 90\% credible level.