Possible Constraints on the Density Dependence of the Nuclear Symmetry Energy from Quasiperiodic Oscillations in Soft Gamma Repeaters

TL;DR

This study uses quasiperiodic oscillations in soft gamma repeaters to constrain the density dependence of nuclear symmetry energy, specifically the parameter L, providing bounds based on neutron star oscillation models.

Contribution

It introduces a method to constrain the symmetry energy parameter L using observed QPOs and torsional oscillation models in neutron star crusts, refining previous bounds.

Findings

Constrains L to be ≥ 47.4 MeV based on fundamental torsional oscillations.

Reproduces observed frequencies with L between 101.1 and 131.0 MeV.

Alternative interpretation suggests L between 58.0 and 85.3 MeV, aligning with other empirical data.

Abstract

We systematically examine the fundamental frequencies of shear torsional oscillations in neutron star crusts in a manner that is dependent on the parameter $L$ characterizing the poorly known density dependence of the symmetry energy. The identification of the lowest quasiperiodic oscillation (QPO) among the observed QPOs from giant flares in soft-gamma repeaters as the $/ell=2$ fundamental torsional oscillations enables us to constrain the parameter $L$ as $L/ge 47.4$ MeV, which is the most conservative restriction on $L$ obtained in the present work that assumes that the mass and radius of the flaring neutron stars range 1.4-1.8 $M_{/odot}$ and 10-14 km. Next, we identify one by one a set of the low-lying frequencies observed in giant flares as the fundamental torsional oscillations. The values of $L$ that can reproduce all the observed frequencies in terms of the torsional oscillations coupled with a part of dripped neutrons via entrainment effects are then constrained as 101.1 MeV $/le L /le$ 131.0 MeV. Alternatively, if only the second lowest frequency observed in SGR 1806$-$20 has a different origin, one obtains relatively low $L$ values ranging 58.0 MeV $/le L /le$ 85.3 MeV, which seem more consistent with other empirical constraints despite large uncertainties.