Induced Scattering of Fast Radio Bursts in Magnetar Magnetospheres

TL;DR

This work addresses whether FRB radiation can escape magnetar magnetospheres or is attenuated by induced scattering in a strongly magnetized $e^{\pm}$ plasma. It combines a kinetic theory framework—distinguishing neutral and charged density-fluctuation modes and their linear growth rates—with PIC simulations to validate the theory and explore nonlinear evolution. The study shows that induced scattering typically enters a linear growth stage, but nonlinear evolution can lead to either full attenuation or saturation, yielding partial scattering and potential escape depending on plasma density and FRB energy; a critical multiplicity $\mathcal{M}_{\mathrm{crit}}$ governs the full vs partial scattering boundary. Applying the framework to GHz FRBs in magnetar magnetospheres yields a regime map indicating most FRBs can escape in many parameter regimes, while certain high-density conditions near magnetars or during giant flares naturally produce strong attenuation, potentially explaining observed FRB diversity and the lack of associated FRBs with some X-ray bursts.

Abstract

We investigate induced Compton/Brillouin scattering of electromagnetic waves in magnetized electron and positron pair plasma by verifying kinetic theory with Particle-in-Cell simulations. Applying this to fast radio bursts (FRBs) in magnetar magnetospheres, we find that the scattering--although suppressed by the magnetic field--inevitably enters the linear growth stage. The subsequent evolution bifurcates: full scattering occurs when the density exceeds a critical value, whereas below it the scattering saturates and the FRB can escape. This eases the tension with observations of compact emission regions and may explain the observed diversity, including the presence or absence of FRBs associated with X-ray bursts.

Figures (4)

• Figure 1: Schematic illustration of neutral-mode induced scattering of an FRB in a magnetar magnetosphere.
• Figure 2: Maximum linear growth rate of induced scattering driven by the neutral mode for a monochromatic incident wave as a function of the incident wave amplitude $\eta^{\mathrm{circ}}_{\mathrm{inci}}$. Red dots denote the PIC results. The blue solid curve is the numerical solution of the dispersion relation of the neutral mode (see Eq. (21) of 2025arXiv251012869N). The orange dashed and red dash-dotted curves are the analytic ICS and SBS in Eq. \ref{['eq:growth_rate_neutral_matome_Brillouin_letter']}, respectively. The purple dotted curve shows the ICS–SBS transition amplitude obtained from Eq. \ref{['eq:transition_point_for_neutral_mode_letter']} rewritten using Eq. \ref{['eq:definition_of_eta_letter']}.
• Figure 3: Time evolution of the forward and backward components of the EM wave energy, normalized by the initial incident wave energy. The horizontal axis is time normalized by $\omega_{0}$, and the vertical axis is the EM wave energy Fourier-transformed along $\bm{B}_0$ (see 2026arXiv260101169K for the detailed treatment of the Fourier transformation). The forward right-handed circularly polarized Alfvén waves (orange dashed) are initially injected, and the scattered components (blue dot-dashed) are the backward right-handed circularly polarized Alfvén waves generated by induced scattering. The green solid curve shows the total EM wave energy, and the black dashed line indicates the slope corresponding to the maximum linear ICS growth rate of the neutral mode in Eq. \ref{['eq:growth_rate_neutral_matome_Brillouin_letter']}.
• Figure 4: Regime map of induced scattering for GHz FRB pulses propagating through a dipolar magnetar magnetosphere. The horizontal axis shows the multiplicity $\mathcal{M}$, and the top axis shows the corresponding magnetization parameter $\sigma_B$. The vertical axis shows the FRB Poynting luminosity $L$. Colored/shaded regions indicate partial scattering (red), full scattering (blue hatched), and the relativistic regime $\eta>1$ (gray). The dominant process in each region is labeled as “Neutral ICS,” “Charged ICS,” or “(Neutral) SBS”. The black curve marks the boundary between partial and full scattering. The parameters used in this figure are listed in the panel.