Jens Mahlmann

Electromagnetic fireworks in the striped magnetar wind

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Fast radio bursts (FRBs) are millisecond blasts of GHz frequency with a complex spectral imprint of their cosmic origin. They are commonly observed from highly magnetized neutron stars, so-called magnetars. The exact origin of FRBs is still not known, and our model examines one possible mechanism to generate them. A very strong magnetic pulse propagates into the striped magnetar wind and compresses the current sheet separating regions of different magnetic field polarity. The strong compression triggers the dynamic rearrangement – reconnection – of magnetic field lines. During this process, magnetic islands form by accumulating strong magnetic flux and plasma density. Mergers of such islands induce strong antenna-like currents that emit high-frequency fast magnetosonic waves which can ultimately convert into the observable radio signal.

Research summary

  • A fraction of the reconnected magnetic field energy (0.2%) is converted into packets of high-frequency fast magnetosonic waves which can escape from the magnetar wind as radio emission.
  • The peak frequency of the outgoing radiation depends on the amplitude of the low-frequency compression pulse and the strength of synchrotron cooling in the reconnection layer. For typical conditions close to extragalactic magnetars, the injected fast magnetosonic waves can reach GHz frequencies, sufficient to explain bright FRBs.
  • The hierarchy during mergers of larger and larger magnetic islands induces a downward frequency drift of the generated fast wave signal and short-duration sub-structures. Such features are also registered in FRB observations.

Visualizing science

Different phases of the low-frequency pulse-current sheet interaction in a large system. The pulse propagates upwards into the upstream. We display the current density (bottom panel), as well as the out-of-plane electric field of the emerging fast wave signal (top panel). The compression triggers reconnection in the otherwise stable current sheet. Small magnetic islands (plasmoid) appear and merge to form hierarchically larger ones under the emission of a firework of high-frequency fast magnetosonic waves.
Plasma frequency (bottom panel) and plasma density (top panel) during the low-frequency pulse-current sheet interaction in a large system. High particle densities accumulate in merging magnetic islands and a complex plasma structure develops in the wake of the low-frequency pulse.

Collaborative results

Mahlmann, J. F., Philippov, A. A., Levinson, A., Spitkovsky, A., & Hakobyan, H. (2022). Electromagnetic fireworks: Fast radio bursts from rapid reconnection in the compressed magnetar wind. The Astrophysical Journal Letters, 932(2), L20, https://doi.org/10.3847/2041-8213/ac7156.