Combined Constraints on the Equation of State of Dense Neutron-rich Matter from Terrestrial Nuclear Experiments and Observations of Neutron Stars

Abstract

Within the parameter space of equation of state (EOS) of dense neutron-rich matter limited by existing constraints mainly from terrestrial nuclear experiments, we investigate how the neutron star maximum mass M max > 2.01 ± 0.04 M ⊙ , radius 10.62 < R 1.4 < 12.83 km and tidal deformability Λ 1.4 ≤ 800 of canonical neutron stars all together constrain the EOS of dense neutron-rich nucleonic matter. While the 3-D parameter space of K sym (curvature of nuclear symmetry energy), J sym and J 0 (skewness of the symmetry energy and EOS of symmetric nuclear matter, respectively) are narrowed down significantly by the observational constraints, more data are needed to pin down the individual values of K sym , J sym and J 0 with quantified uncertainties. The J 0 largely controls the maximum mass of neutron stars. While the EOS with J 0 = 0 is sufficiently stiff to support neutron stars as massive as 2.37 M ⊙ , to support the hyperthetical ones as massive as 2.74 M ⊙ (composite mass of GW170817) requires J 0 to be larger than its currently known maximum value of about 400 MeV and beyond the causality limit. The upper limit on the tidal deformability of Λ 1.4 = 800 from the recent observation of GW170817 is found to provide upper limits on some EOS parameters consistent with but far less restrictive than the existing constraints of other observables studied.

Neutron star mergers and how to study them

“…E 0 and E sym near the saturation density can be described with characteristic parameters of the EOS. Some of these parameters are reasonably well determined from terrestrial experiments and can be used to inform astrophysical inferences, e.g., excluding a Stable NS remnant for GW170817 (Zhang et al 2018b). Otherwise, improved measurements on the properties of NSs and understanding of the NS EOS can provide new understanding on the nuclear symmetry energy at supranuclear densities (Li et al 2019).…”

“…A full understanding of such densities requires the intersection of nuclear physics and astrophysics. Following discussions in Zhang et al (2018b), we can Coughlin et al (2019b), copyright by the authors approximate the energy per nucleon Eðq; dÞ % E 0 ðqÞ þ E sym ðqÞd 2 , with the isospin asymmetry d ¼ ðq n À q p Þ=q and E 0 ðqÞ the energy in symmetric matter. E sym captures the effects of the neutron-richness of the system.…”

Neutron-star tidal deformability and equation-of-state constraints

“…Various studies have empirically identified correlations between various combinations of parameters that characterize the symmetry energy and its density dependence with the neutron star radius and tidal parameters [246,[266][267][268][269] or terrestrial experimental results [239,[270][271][272], as they all are linked to the low-density behavior of the equation of state around (twice) saturation. The tidal constraints from GW170817, and more recently radius constraints from NICER have been used to examine implications for the symmetry energy [273][274][275][276][277][278][279][280][281][282] as well as potential systematics in the mapping [283].…”

GW170817: Constraining the nuclear matter equation of state from the neutron star tidal deformability

“…In earlier studies it was found that correlations between the various properties of NS and nuclear matter EoS parameters are significantly affected when a more diverse set of models are employed [25,26]. Recently, astrophysical observations of NS, in particular, the maximum mass, the radius of a canonical 1.4 M ⊙ NS, and the tidal deformability, have been used to constrain various parameters of the EoS [27]. However, within their assumptions, they found that the tidal deformability obtained from GW170817 is not very restrictive.…”