[Paper Review] Dense Matter in Neutron Star: Lessons from GW170817

Sarmistha Banik, Debades Bandyopadhyay|arXiv (Cornell University)|Dec 28, 2017

Pulsars and Gravitational Waves Research4 citations

TL;DR

This paper investigates dense matter equations of state (EoS) in neutron stars using multimessenger data from the GW170817 neutron star merger. Applying the density-dependent relativistic hadron field theory, it finds that the BHB Λφ EoS—incorporating Λ hyperons—is most consistent with the observed upper limit on neutron star maximum mass (~2.16 M⊙) and tidal deformability, making it the favored EoS among tested models.

ABSTRACT

Neutron star merger event GW170817 sets an upper limit on the maximum mass of non-rotating neutron stars. Consequently, this event puts strong constraints on the dense matter equation of state (EoS). A comparative study of dense matter equations of state (EoSs) is presented here. It is found that the $Λ$ hyperon EoS BHB$Λϕ$ (Banik, Hempel $\&$ Bandyopadhyay 2014) constructed within the framework of the density dependent hadron field theory is favoured.

Motivation & Objective

• To constrain the equation of state (EoS) of dense matter in neutron stars using multimessenger data from GW170817.
• To evaluate the consistency of various EoS models—especially hyperon-containing ones—with observational limits on neutron star maximum mass.
• To determine which EoS best satisfies both the upper limit on maximum mass (2.16 M⊙) and tidal deformability from GW170817.
• To assess the role of hyperons, particularly Λ hyperons, in softening the EoS and affecting neutron star structure.
• To compare the BHB Λφ EoS with other widely used EoSs (e.g., LS220, APR4, SFHo, DD2) in light of current astrophysical constraints.

Proposed method

• Uses the density-dependent relativistic hadron field theory (DDRH) to construct the BHB Λφ EoS, incorporating Λ hyperons and medium effects.
• Applies an extended nuclear statistical equilibrium (NSE) model to describe non-uniform matter at sub-saturation densities, including nuclei, nucleons, and leptons.
• Employs relativistic mean-field (RMF) approximation for nucleon interactions and accounts for Coulomb, screening, and excluded volume effects.
• Calculates the canonical partition function for inhomogeneous matter, including contributions from unbound nucleons, nuclei, and Coulomb interactions.
• Compares EoS predictions against observational constraints: the lower limit on maximum mass (2.01 M⊙) from PSR J09708+0534 and the upper limit (2.16 M⊙) derived from GW170817.
• Evaluates tidal deformability from GW170817 to further constrain EoS models, excluding stiff or inconsistent EoSs.

Experimental results

Research questions

• RQ1 Which equations of state for dense neutron star matter are consistent with the upper limit on the maximum mass of non-rotating neutron stars derived from GW170817?
• RQ2 How do the nuclear matter properties (saturation density, symmetry energy, incompressibility, slope) of different EoSs compare with experimental constraints?
• RQ3 What is the impact of including Λ hyperons on the stiffness and maximum mass of neutron star EoSs?
• RQ4 Which EoS best matches both the tidal deformability observed in GW170817 and the mass constraints from pulsar observations?
• RQ5 Why is the BHB Λφ EoS found to be favored over other hyperon-containing or nucleon-only EoSs like DD2, APR4, or LS220?

Key findings

• The BHB Λφ EoS, constructed within the DDRH framework with Λ hyperons, is consistent with the upper limit on maximum neutron star mass of 2.16 M⊙ from GW170817.
• The EoS is also consistent with the tidal deformability inferred from GW170817, which excludes stiffer EoSs like LS220, APR4, and H4.
• Among the tested EoSs, only BHB Λφ, SFHo, and DD2 have nuclear matter properties close to experimental values, but DD2 is ruled out by the mass limit.
• The LS220, MS1, APR4, and H4 EoSs are inconsistent with experimental constraints on incompressibility (K) or symmetry energy slope (L), leading to unphysical stiffness or softness.
• The BHB Λφ EoS predicts a maximum mass of 2.11 M⊙, falling within the 2.01–2.16 M⊙ observational window.
• The study concludes that the inclusion of Λ hyperons in the BHB Λφ EoS leads to a softening effect that matches observations without violating mass or tidal constraints.