The synchrotron maser emission from relativistic shocks in Fast Radio Bursts: 1D PIC simulations of cold pair plasmas

Abstract: The emission process of Fast Radio Bursts (FRBs) remains unknown. We investigate whether the synchrotron maser emission from relativistic shocks in a magnetar wind can explain the observed FRB properties. We perform particle-in-cell (PIC) simulations of perpendicular shocks in cold pair plasmas, checking our results for consistency among three PIC codes. We confirm that a linearly polarized X-mode wave is self-consistently generated by the shock and propagates back upstream as a precursor wave. We find that at magnetizations $\sigma\gtrsim 1$ (i.e., ratio of Poynting flux to particle energy flux of the pre-shock flow) the shock converts a fraction $f_\xi' \approx 7 \times 10^{-4}/\sigma^2$ of the total incoming energy into the precursor wave, as measured in the shock frame. The wave spectrum is narrow-band (fractional width $\lesssim 1-3$), with apparent but not dominant line-like features as many resonances concurrently contribute. The peak frequency in the pre-shock (observer) frame is $\omega^{\prime \prime}{\rm peak} \approx 3 \gamma{\rm s | u} \omega_{\rm p}$, where $\gamma_{\rm s|u}$ is the shock Lorentz factor in the upstream frame and $\omega_{\rm p}$ the plasma frequency. At $\sigma\gtrsim1$, where our estimated $\omega''_{\rm peak}$ differs from previous works, the shock structure presents two solitons separated by a cavity, and the peak frequency corresponds to an eigenmode of the cavity. Our results provide physically-grounded inputs for FRB emission models within the magnetar scenario.

Synchrotron Maser Emission from Relativistic Shocks in Fast Radio Bursts

The paper titled "The synchrotron maser emission from relativistic shocks in Fast Radio Bursts: 1D PIC simulations of cold pair plasmas" by Illya Plotnikov and Lorenzo Sironi offers an insightful analysis of synchrotron maser emission in the context of Fast Radio Bursts (FRBs). The study employs one-dimensional particle-in-cell (PIC) simulations, exploring the behavior of perpendicular shocks in cold pair plasmas—specifically within magnetar winds. This work aims to connect the theoretical underpinnings of synchrotron maser emission at relativistic shocks with the observed properties of FRBs.

Key Findings

1. Emission Efficiency and Energetics: The simulations reveal that for magnetization $\sigma \gtrsim 1$, the shock converts a fraction $f_\xi' \approx 7 \times 10^{-4}/\sigma^2$ of the total incoming energy into the emissive precursor wave, as measured in the shock rest frame (SRF). This decrease in efficiency with increased magnetization aligns with expectations for magnetically dominated shock regimes.

2. Precursor Wave Characteristics: The precursor wave in the shock front is identified as linearly polarized, propagating upstream, and characterized by a narrow spectral width ($\Delta \omega/\omega_{\rm peak} \lesssim 1-3$). The typical emission observed is consistent with extraordinary mode (X-mode) polarization.

3. Peak Frequency Scaling: The peak frequency of the wave emission in the upstream (observer) frame scales with $\omega^{\prime \prime}{\rm peak} \approx 3 \gamma{\rm s | u} \omega_{\rm p}$, where $\gamma_{\rm s|u}$ is the upstream shock Lorentz factor, and $\omega_{\rm p}$ is the plasma frequency. This finding corrects earlier assumptions about the shock structure where the frequency scaling diverges from previous analytical estimates.

4. Role of Shock Structure: At high magnetization ($\sigma \gtrsim 1$), the formation of density and magnetic field cavities significantly impacts the shock structure and its emissive properties, suggesting that dimensional effects in the shock front are instrumental in shaping the electromagnetic emission.

5. Implications for FRB Models: The results provide robust physical inputs for FRB models within the magnetar scenario. Specifically, the efficiently emitted coherent waves and their spectral characteristics can potentially account for the high brightness temperatures and polarizations observed in FRBs.

Implications for Future Research

This study contributes significantly to advancing our understanding of FRBs and the role of relativistic shocks in their formation. The implications are threefold:

1. Refinement of FRB Models: By offering a quantitative foundation for the conversion of kinetic energy to coherent radio emission via shock-driven synchrotron masers, this work refines existing FRB models tied to magnetars.

2. Broadening Shock Studies: The unique results highlighting the role of solitons and cavities in shock structures warrant further exploration. Extending beyond 1D PIC simulations to multidimensional models could yield a deeper understanding of spatial effects in relativistic scenarios.

3. Comparative Analysis with Observations: As simulations become more representative of actual astrophysical conditions, comparing these results against FRB observations, including polarization and spectral properties, will be crucial for corroborating the theoretical predictions.

In summary, this paper offers a comprehensive investigation into synchrotron maser emission mechanisms as related to FRBs, providing a valuable framework to interpret observational data and guide future collaborative research efforts in high-energy astrophysics.