Modulating the magnetosphere of magnetars by internal magneto-elastic oscillations

TL;DR

This paper models how internal magneto-elastic oscillations in magnetars influence their magnetospheres, revealing that such oscillations can modulate the external magnetic field and induce time-dependent twists, especially near the poles.

Contribution

It introduces a coupling framework between internal magneto-elastic oscillations and the external magnetosphere, showing how internal dynamics affect surface magnetic field perturbations and magnetospheric configurations.

Findings

Oscillations modulate the magnetosphere via surface magnetic field perturbations.

Magnetic field modulations are symmetric with respect to the equator.

Limited Alfvén wave excitation occurs near the poles.

Abstract

We couple internal torsional, magneto-elastic oscillations of highly magnetized neutron stars (magnetars) to their magnetospheres. The corresponding axisymmetric perturbations of the external magnetic field configuration evolve as a sequence of linear, force-free equilibria that are completely determined by the background magnetic field configuration and by the perturbations of the magnetic field at the surface. The perturbations are obtained from simulations of magneto-elastic oscillations in the interior of the magnetar. While such oscillations can excite travelling Alfv\'en waves in the exterior of the star only in a very limited region close to the poles, they still modulate the near magnetosphere by inducing a time-dependent twist between the foot-points of closed magnetic field lines that exit the star at a polar angle $\gtrsim 0.19\,$rad. Moreover, we find that for a dipole-like background magnetic field configuration the magnetic field modulations in the magnetosphere, driven by internal oscillations, can only be symmetric with respect to the equator. This is in agreement with our previous findings, where we interpreted the observed quasi-periodic oscillations in the X-ray tail of magnetar bursts as driven by the family of internal magneto-elastic oscillations with symmetric magnetic field perturbations.