Kinking flux tubes and global eruptions: The 3D dynamics of strongly twisted magnetar magnetospheres

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The origin of hard X-ray outbursts from magnetars, highly magnetized neutron stars, is still unknown. These outbursts range from frequent faint events to the most energetic Giant Flares and span several orders of magnitude in energy. Our work explores how different instabilities of the magnetar magnetosphere can release energy that shines in X-rays. They can also launch waves that affect the outer magnetosphere, creating X-ray bursts through other channels or possibly fast radio bursts via reconnection. Besides globally erupting magnetospheres that release the twist energy of a flux tube in extended reconnection events, we find a new instability that operates on a more confined scale. Kink-like helical eruptions can dissipate energy without significantly rearranging the topology of the magnetosphere. While global eruptions eject powerful energy bubbles that propagate to the outer magnetosphere as nonlinear structures, kinking flux tubes seed fast magnetosonic waves. These waves travel across magnetic field lines until they become electrically dominated and possibly form strong shocks that can drive further X-ray activity.

Research summary

  • During an eruption, around 25% of the twist energy is dissipated. This energy was injected into the magnetosphere by twisting the footpoints of magnetic field lines. We suggest that this release of free energy drives X-ray flaring activity.
  • In 2D, twists extending over the entire stellar surface can easily dissipate Giant Flare energies and always erupt on large scales. In 3D, the available free energy is generally lower, and dissipation events reach 0.1% to 1% of the initial dipole energy. Such 3D events could power a wide range of fainter X-ray bursts.
  • Global eruptions eject energetic nonlinear structures that carry energy to the outer magnetosphere in Poynting-flux-loaded blobs, reaching up to 1% of the dipole energy in our models. Confined, kink-like eruptions do not affect the large-scale magnetospheric structure. However, they seed low-amplitude fast magnetosonic waves that become electrically dominated at around 100 stellar radii.

Visualizing science

Axisymmetric magnetospheric twist triggering a large-scale reconnection event and a global rearrangement of the magnetosphere. A constant twist is induced in a region centered at θ = 45° with an extension of 0.05π.
3D visualization of field line dynamics in the magnetar magnetosphere. A constant twist is induced in a region centered at θ = 45° with an extension of 0.05π. The subsequent eruptions stay confined to the inner magnetosphere. Volume-filling colors show the conserved parallel current. Inset plots show the poloidal magnetic field as field lines and the toroidal magnetic field in color, in a meridional slice centered on the induced twist.
3D visualization of field line dynamics in the magnetar magnetosphere. A constant twist is induced in a region centered at θ = 55° with an extension of 0.1π. The subsequent eruptions open up the magnetosphere on a global scale. Volume-filling colors show the conserved parallel current. Inset plots show the poloidal magnetic field as field lines and the toroidal magnetic field in color, in a meridional slice centered on the induced twist.

Collaborative results

Mahlmann, J. F., Philippov, A. A., Mewes, V., Ripperda, B., Most, E. R., & Sironi, L. (2023). Three-dimensional dynamics of strongly twisted magnetar magnetospheres: Kinking flux tubes and global eruptions. The Astrophysical Journal Letters, 947, L34, https://doi.org/10.3847/2041-8213/accada