Electromagnetic fireworks in the striped magnetar wind

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Fast radio bursts (FRBs) are millisecond flashes of GHz radio waves that carry a complex spectral imprint of their cosmic origin. Highly magnetized neutron stars, so-called magnetars, are leading candidates for their sources, and in 2020 an FRB was detected from a magnetar in our own galaxy. The exact origin of FRBs is still unknown, 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 that separates regions of opposite magnetic polarity. The strong compression triggers reconnection, a dynamic rearrangement of magnetic field lines. During this process, magnetic islands form and accumulate 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

  • About 0.2% of the reconnected magnetic field energy 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 on the strength of synchrotron cooling in the reconnection layer. For typical conditions near extragalactic magnetars, the injected fast magnetosonic waves can reach GHz frequencies, sufficient to explain bright FRBs.
  • The hierarchical merging of ever larger magnetic islands induces a downward frequency drift of the generated fast wave signal and short-duration substructures. Such features are also seen in FRB observations.

Visualizing science

Different phases of the interaction between a low-frequency pulse and a current sheet in a large system. The pulse propagates upward into the upstream plasma. The bottom panel shows the current density, and the top panel shows the out-of-plane electric field of the emerging fast wave signal. The compression triggers reconnection in the otherwise stable current sheet. Small magnetic islands, called plasmoids, appear and merge into hierarchically larger ones, emitting a firework of high-frequency fast magnetosonic waves.
Plasma frequency in the bottom panel and plasma density in the top panel during the interaction between a low-frequency pulse and a current sheet in a large system. High particle densities accumulate in merging magnetic islands, and a complex plasma structure develops in the wake of the 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