Electromagnetic fireworks: Fast radio bursts from rapid reconnection in the compressed magnetar wind

One scenario for the generation of fast radio bursts (FRBs) is magnetic reconnection in a current sheet of the magnetar wind. Compressed by a strong magnetic pulse induced by a magnetar flare, the current sheet fragments into a self-similar chain of magnetic islands. Time-dependent plasma currents at their interfaces produce coherent radiation during their hierarchical coalescence. We investigate this scenario using 2D radiative relativistic particle-in-cell simulations to compute the efficiency of the coherent emission and to obtain frequency scalings. Consistent with expectations, a fraction of the reconnected magnetic field energy, $f\sim 0.002$, is converted to packets of high-frequency fast magnetosonic waves which can escape from the magnetar wind as radio emission. In agreement with analytical estimates, we find that magnetic pulses of $10^{47}\text{erg}\;\text{s}^{-1}$ can trigger relatively narrowband GHz emission with luminosities of approximately $10^{42}\text{erg}\;\text{s}^{-1}$, sufficient to explain bright extragalactic FRBs. The mechanism provides a natural explanation for a downward frequency drift of burst signals, as well as the $\sim 100\;\text{ns}$ substructure recently detected in FRB 20200120E.