Possible constraints on the density dependence of the nuclear symmetry energy from quasi-periodic oscillations in soft gamma repeaters

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 observedQPOs from giant flares in soft-gamma repeaters as the l = 2 fundamental torsional oscillations enables us to constrain the parameter L as L ≤ 47.4MeV, 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 1.4-1.8M⊙ 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 ≤ L ≤ 131.0MeV. 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 ≤ L ≤ 85.3MeV, which seem more consistent with other empirical constraints despite large uncertainties. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.