Superluminal Wave Activation at Relativistic Magnetized Shocks

TL;DR

This paper demonstrates through theory and simulations that superluminal O-modes can be activated at relativistic magnetized shocks, potentially explaining the emission mechanism of fast radio bursts in magnetar environments.

Contribution

It validates the superluminal wave activation mechanism using pair-plasma theory and particle-in-cell simulations, revealing conditions for wave conversion at shocks.

Findings

Superluminal wave activation occurs when upstream perturbation frequency exceeds downstream plasma frequency.

Simulations confirm wavenumber and frequency jumps across shocks for high-frequency upstream waves.

Downstream plasma frequency acts as a high-pass filter for superluminal O-modes.

Abstract

Fast radio bursts (FRBs) are extremely energetic radio transients, some are generated in magnetar magnetospheres and winds. Despite a growing number of observations, their emission mechanisms remain elusive. It has recently been proposed that Alfv\'enic perturbations can convert into superluminal O-modes at magnetized shocks and propagate downstream as a radio signal. We validate this superluminal wave activation mechanism using pair-plasma theory and particle-in-cell simulations. Theory predicts two different downstream modes: nonpropagating Alfv\'enic perturbations and propagating superluminal O-modes. Superluminal wave activation occurs if the frequency of upstream perturbations in the shock frame exceeds the downstream plasma frequency. 1D particle-in-cell simulations confirm wavenumber and frequency jumps across the shock for upstream perturbations with frequencies well above the plasma frequency. Our simulations model both monochromatic upstream waves and broadband spectra with the downstream plasma frequency acting like a high-pass filter for superluminal O-modes. We discuss implications for FRB generation in relativistic magnetized winds.