Magnetic Stresses & Fast Radio Bursts

In this letter (Dehman et al. 2020, ApJL 902, L32), we investigated the long-term evolution of magnetic fields in neutron stars and their role in powering magnetar activity. Using magneto-thermal simulations, we demonstrated that the gradual reconfiguration of complex internal magnetic fields generates strong magnetic stresses within the solid crust. These stresses arise from the competition between the evolving magnetic field, which tends to reach a lower-energy configuration, and the elastic resistance of the crust, which anchors the field lines. When the accumulated stress locally exceeds the crust’s yield limit, a sudden rearrangement occurs—releasing stored magnetic energy and producing transient emission events.

This mechanism naturally explains the episodic nature of magnetar outbursts and provides a physical link between the internal magnetic evolution and the short-timescale phenomena observed across the electromagnetic spectrum. In particular, the rapid magnetic energy release and subsequent reconfiguration could act as the trigger for fast radio bursts (FRBs) associated with magnetars, connecting the long-term magnetic evolution of magnetars to their most extreme and transient manifestations.

Our results indicate that magnetar activity is primarily governed by the magnetic energy stored in the crust rather than by the surface dipole field. While earlier studies associated outburst rates and intensities with the spin-down–inferred dipole, our simulations show that activity rates can vary by orders of magnitude depending on the structure of the internal field. Even neutron stars with moderate dipoles can accumulate substantial magnetic energy in buried toroidal and higher-order multipolar components. The evolution and reconfiguration of this internal field generate magnetic stresses that trigger crustal failures. Consequently, the total crustal magnetic energy provides a far more reliable predictor of bursting activity than the dipole alone, explaining outbursts in low-dipole-field magnetars and some apparently ordinary pulsars. These results highlight that magnetar activity is controlled by the hidden internal energy reservoir, not merely by the observable dipole component.

Our work highlights a connection between magnetar crustal activity and fast radio bursts (FRBs). Only a small fraction of magnetar crustal failures need to be observable to explain the FRBs detected across the sky. Similarly, young magnetars in our Galaxy show X-ray outbursts at rates consistent with our predictions. This suggests a common origin for these events: crustal triggers within magnetars, which can produce both Galactic outbursts and the fastest, most energetic radio bursts.