Possible identifications of newly observed magnetar quasi-periodic oscillations as crustal shear modes

Abstract

Quasi-periodic oscillations (QPOs) discovered in soft-gamma repeaters (SGRs) are expected to help us to study the properties of matter in neutron stars. In earlier investigations, we identified the QPOs of frequencies below ∼100 Hz observed in giant flares of SGR 1806−20 and SGR 1900+14 as the crustal torsional oscillations. For this purpose, we calculated the frequencies of the fundamental torsional oscillations with various angular indices ℓ, by changing the stellar mass and radius. In this work, we try to explain the additional QPO frequencies recently reported by Huppenkothen et al. (2014a, 2014b) within the same framework as before except that we newly take into account the effect of electron screening, which acts to decrease the frequencies by a small amount. Those QPOs were discovered in two different SGRs, i.e., SGR 1806−20 and SGR J1550−5418. Then, we find that the newly observed QPO frequency in SGR 1806−20 can be still identified as one of the frequencies of the fundamental torsional oscillations, while those in SGR J1550−5418 can also be explained in terms of the torsional oscillations although the relevant angular indices are difficult to identify.