Effect of superfluidity on neutron star crustal oscillations

TL;DR

This paper investigates how superfluid neutrons in neutron star crusts influence torsional oscillation frequencies, leading to tighter constraints on the symmetry energy density derivative from observed giant flare oscillations.

Contribution

It introduces the effect of superfluidity on crustal oscillation frequencies and refines the constraints on the symmetry energy derivative using observational data.

Findings

Superfluid neutrons reduce enthalpy density, increasing oscillation frequencies.

Constraints on the symmetry energy derivative L are tightened to 100-130 MeV.

Superfluidity effects are significant for interpreting neutron star oscillations.

Abstract

We consider how superfluidity of dripped neutrons in the crust of a neutron star affects the frequencies of the crust's fundamental torsional oscillations. A nonnegligible superfluid part of dripped neutrons, which do not comove with nuclei, act to reduce the enthalpy density and thus enhance the oscillation frequencies. By assuming that the quasi-periodic oscillations observed in giant flares of soft gamma repeaters arise from the fundamental torsional oscillations and that the mass and radius of the neutron star is in the range of 1.4 < M/M_SUN < 1.8 and 10 km < R < 14 km, we constrain the density derivative of the symmetry energy as 100 MeV < L < 130 MeV, which is far severer than the previous one, L > 50 MeV, derived by ignoring the superfluidity.