Explaining magnetar QPOs via nonlinear coupling

The quasi-periodic oscillations (QPOs) observed in the tails of magnetar giant gamma-ray flares have long been interpreted as normal oscillation modes of these stars. However, most studies modelling QPOs have neglected some key features in the analyses of the signals, namely that QPOs appear to be detectable only intermittently and exhibit drifts in their frequencies. These are typical characteristics of nonlinear mode coupling, where, at leading order, the modes couple and evolve collectively as triplets. We show that a model which incorporates such nonlinear effects is able to explain the observed QPO phenomenology and provides a way to infer details of the magnetar's internal magnetic field geometry.