[Paper Review] Constraining the equation of state with gravitational wave observation

Rana Nandi, Prasanta Char+1 more|arXiv (Cornell University)|Sep 19, 2018

Pulsars and Gravitational Waves Research2 references4 citations

TL;DR

This paper constrains the nuclear equation of state using gravitational wave data from GW170817 and neutron star mass measurements, analyzing relativistic mean-field models against tidal deformability and neutron skin thickness. It finds only a few stiff equations of state with soft symmetry energy survive constraints, leading to an upper limit of R(1.4M⊙) < 12.9 km for a 1.4 solar mass neutron star.

ABSTRACT

The first detection of gravitational waves from binary neutron star merger event GW170817 has started to provide important new constraints on the nuclear equation of state at high density. The tidal deformability bound of GW170817 combined with the observed 2M_solar neutron star poses serious challenge to theoretical formulations of realistic equation of state. We analyze a fully comprehensive set of relativistic nuclear mean-field theories by confronting with the observational bounds and the measured neutron-skin thickness. We find only few models can withstand these bounds which predict a stiff overall equation of state but with a soft neutron-proton symmetry energy. Two possible indications are proposed: Circumstantial evidence of hadron-quark phase transition inside the star and new parametrizations that are consistent with ground state properties of finite nuclei and observational bounds. Based on extensive analysis of these sets, an upper limit on the radius of a 1.4M_solar neutron star of R(1.4M_solar) < 12.9 km is deduced.

Motivation & Objective

• To constrain the nuclear equation of state at high densities using gravitational wave observations from GW170817.
• To reconcile theoretical models of neutron stars with observed tidal deformability and the 2M⊙ mass limit.
• To assess the consistency of relativistic mean-field models with both nuclear ground-state properties and astrophysical observations.
• To identify viable equations of state that satisfy both neutron skin thickness measurements and gravitational wave constraints.

Proposed method

• A comprehensive set of relativistic nuclear mean-field theories is systematically evaluated against observational bounds.
• Tidal deformability from GW170817 is used as a key constraint on the equation of state.
• Neutron skin thickness measurements from atomic nuclei are incorporated to further restrict model parameters.
• The analysis focuses on models that simultaneously satisfy 2M⊙ mass constraints and tidal deformability limits.
• A Bayesian or likelihood-based comparison is implied to rank models based on consistency with data.
• The radius of a 1.4M⊙ neutron star is derived from surviving models to establish an upper bound.

Experimental results

Research questions

• RQ1 Which relativistic mean-field models of the nuclear equation of state are consistent with GW170817's tidal deformability and the 2M⊙ neutron star mass?
• RQ2 How do neutron skin thickness measurements constrain the symmetry energy and equation of state?
• RQ3 What does the combination of gravitational wave and nuclear data imply about the presence of a hadron-quark phase transition in neutron stars?
• RQ4 Can a stiff overall equation of state coexist with a soft symmetry energy, and what are the implications for neutron star structure?
• RQ5 What is the tightest upper bound on the radius of a 1.4M⊙ neutron star consistent with current observational and theoretical constraints?

Key findings

• Only a small subset of relativistic mean-field models survive the combined constraints of GW170817's tidal deformability and the 2M⊙ neutron star mass.
• These surviving models predict a stiff overall equation of state but with a soft neutron-proton symmetry energy.
• Circumstantial evidence suggests a possible hadron-quark phase transition within neutron stars.
• The models indicate a new class of parametrizations consistent with both finite nucleus ground states and astrophysical observations.
• An upper limit of R(1.4M⊙) < 12.9 km is established for the radius of a 1.4 solar mass neutron star.