On the nature of QPO in the tail of SGR giant flares

TL;DR

This paper presents a model explaining the quasiperiodic oscillations in magnetar giant flare tails as resulting from magnetospheric modulation caused by surface torsional oscillations, accounting for observed spectral features.

Contribution

It introduces an axisymmetric analytical model linking surface torsional oscillations to magnetospheric modulation and QPOs in magnetar flare tails, a novel approach in this context.

Findings

Neutron star surface oscillation amplitude needed is about 1% of NS radius.

Optical depth anisotropy explains phase-dependent QPO observations.

Model reproduces key features of observed power spectra.

Abstract

A model is presented for the quasiperiodic component of magnetar emission during the tail phase of giant flares. The model invokes modulation of the particle number density in the magnetosphere. The magnetospheric currents are modulated by torsional motion of the surface and we calculate that the amplitude of neutron star surface oscillation should be ~1% of the NS radius in order to produce the observed features in the power spectrum. Using an axisymmetric analytical model for structure of the magnetosphere of an oscillating NS, we calculate the angular distribution of the optical depth to the resonant Compton scattering. The anisotropy of the optical depth may be why QPO are observed only at particular rotational phases.