Absorption of strong electromagnetic waves in magnetized pair plasmas

TL;DR

The paper addresses how strong electromagnetic pulses propagate through magnetized pair plasmas in the context of FRBs. It reframes stochastic heating as synchrotron absorption and derives a lab-frame resonance condition that confines absorption to the interval $\omega_{\rm B}/a_0<ω<a_0ω_{\rm B}$, robust across different bulk acceleration scenarios. By analyzing no-acceleration, maximal-acceleration, and self-consistent solutions, it shows that absorption occurs only within this window, highlighting the importance of nonlinear wave dynamics. The findings constrain FRB emission models and magnetar environments, emphasizing nonlinear wave-plasma interactions and the need for fully kinetic simulations to test stochastic heating in this regime.

Abstract

We discuss synchrotron absorption of a short electromagnetic pulse that propagates in a cold magnetized pair plasma. We show that the pulse can be absorbed when $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$, where $a_0>1$ is the strength parameter of the pulse, and $ω$ and $ω_{\rm B}$ respectively are the frequency of the wave and the cyclotron frequency in the background magnetic field (all quantities are defined in the reference frame where the particles are at rest before being illuminated by the pulse). The condition $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$ is essentially a generalization of the cyclotron resonance to strong electromagnetic pulses with $a_0>1$. When $ω_{\rm B}/a_0< ω< a_0ω_{\rm B}$, the propagation of electromagnetic waves in a plasma can be very different with respect to the propagation in vacuum because the wave equation is strongly non-linear. Then it is unclear whether the particles are heated stochastically due to synchrotron absorption, as found by studying the motion of a test particle in the field of a vacuum electromagnetic wave. We discuss implications of our results for constraining emission models of fast radio bursts.