Constraining the density dependence of the nuclear symmetry energy from an X-ray bursting neutron star

TL;DR

This paper uses X-ray burst observations of a neutron star to constrain the nuclear symmetry energy's density dependence, specifically providing lower bounds on the parameter L related to nuclear matter properties.

Contribution

It introduces a method to constrain the symmetry energy parameter L using mass-radius data from neutron star X-ray bursts, refining previous bounds.

Findings

L > 110 MeV for K_0=180 MeV

L > 98 MeV for K_0=230 MeV

L > 78 MeV for K_0=360 MeV

Abstract

Neutrons stars lighter than the Sun are basically composed of nuclear matter of density up to around twice normal nuclear density. In our recent analyses, we showed that possible simultaneous observations of masses and radii of such neutron stars could constrain $\eta\equiv(K_0L^2)^{1/3}$, a combination of the incompressibility of symmetric nuclear matter $K_0$ and the density derivative of the nuclear symmetry energy $L$ that characterizes the theoretical mass-radius relation. In this paper, we focus on the mass-radius constraint of the X-ray burster 4U 1724-307 given by Suleimanov et al. (2011). We therefrom obtain the constraint that $\eta$ should be larger than around 130 MeV, which in turn leads to $L$ larger than around 110, 98, 89, and 78 MeV for $K_0=180$, 230, 280, and 360 MeV. Such a constraint on $L$ is more or less consistent with that obtained from the frequencies of quasi-periodic oscillations in giant flares observed in soft-gamma repeaters.