Filamentation of the electromagnetic precursor in relativistic quasi-perpendicular electron-positron shocks

¹ Gran Sasso Science Institute, viale F. Crispi 7, L’Aquila 67100, Italy
² INFN – Laboratori Nazionali del Gran Sasso, via G. Acitelli 22 Assergi 67100, Italy
³ Physics Department, Ben-Gurion University, Be’er-Sheva 84105, Israel
⁴ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 550 W 120th St, New York NY 10027, USA
⁵ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York NY 10010, USA
⁶ Graduate School of System Informatics, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe, Hyogo 657-8501, Japan

^⋆ Corresponding author: emanuele.sobacchi@gssi.it

Received: 11 June 2025
Accepted: 30 July 2025

Abstract

We present a scenario that could explain non-thermal particle acceleration in relativistic quasi-perpendicular electron-positron shocks, such as the termination shock of pulsar wind nebulae. The shock produces a strong electromagnetic precursor that propagates into the upstream plasma, which is initially threaded by a uniform background magnetic field. We show that the filamentation instability breaks the precursor into radiation filaments parallel to the shock normal. The transverse scale of the filaments is of the order of a few plasma skin depths. In the shock frame, the bulk Lorentz factor of the upstream plasma is significantly reduced inside the radiation filaments. Then, the instability produces a relativistic shear flow with strong velocity gradients on kinetic scales. The velocity gradients distort the background magnetic field lines and generate a magnetic field component parallel to the shock normal that reverses across each radiation filament, a configuration that could trigger magnetic reconnection in the upstream plasma. These effects may accelerate particles before the plasma enters the shock.