Preferential Positron Acceleration in Relativistic Magnetized Electron-Positron-Ion Shocks

TL;DR

The paper investigates the origin of high-energy primary positrons by examining relativistic magnetized shocks in electron-positron-ion plasmas using 1D ab initio PIC simulations. It demonstrates a preferential acceleration of positrons via interaction with wakefields generated by upstream precursor waves, interpretable as relativistic E×B motion in the upstream frame. The authors develop a criterion linking wakefield amplitude, magnetization, and bulk Lorentz factor to determine where this mechanism operates, showing peak effectiveness near a total magnetization σ_tot ≈ 0.15 and enhanced gains for ultra-relativistic flows. The findings suggest pulsar winds as plausible sites for producing high-energy positrons and highlight the need for multidimensional and laboratory studies to fully quantify the mechanism's efficiency and applicability.

Abstract

Relativistic shocks are considered efficient accelerators of charged particles and play crucial roles in high-energy astrophysical phenomena, such as gamma-ray bursts and pulsar winds. This study focuses on positron accelerations in magnetized relativistic shocks in electron-positron-ion plasma. Employing one-dimensional ab initio particle-in-cell simulations, we found a preferential positron acceleration through an interaction with the wakefield associated with a precursor wave in the upstream region. Test particle simulations revealed that the selective acceleration occurs for sufficiently large amplitudes of the wakefield. The mechanism can be understood as the relativistic $\boldsymbol{E}\times\boldsymbol{B}$ acceleration formulated in the upstream frame. A theoretical analysis of the positron acceleration in astrophysical contexts is presented, supporting ultra-relativistic shocks in pulsar winds as a primary source for the high-energy positron excess.

Figures (9)

• Figure 1: Overall shock structure at $t=4000\ \omega_{\rm{pe}}^{-1}$ for $\sigma_{\rm{tot}}=0.15$ and $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.2$. From top to bottom, the number density profiles of (a) electron, (b) positron, and (c) ion, (d) the $z$-component of the magnetic field $B_z$, (e) the longitudinal electric field $E_x$, and the (f) electron, (g) positron, and (h) ion phase space densities $x-u_{x\mathrm{s}}$ are shown. All quantities are normalized to the upstream values, and the color scale of the phase-space plots is on a logarithmic scale.
• Figure 2: Enlarged view of Figure \ref{['fig:ModerateSS']} around the wakefield in the upstream region. From top to bottom, shown are (a) $E_x$, electrons phase space densities for (b) $u_{xe^-}$ and (c) $u_{ye^-}$, and positron phase space densities for (d) $u_{xe^+}$ and (e) $u_{ye^+}$ in the same format as Figure \ref{['fig:ModerateSS']}.
• Figure 3: (a) The $z$-component of the magnetic fields and (b) the $x$-component of the electric field for positron fractions of $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.05$ (green), $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.20$ (red), and $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.50$ (blue). $x-x_{\mathrm{sh}}$ indicates the position from the shock front.
• Figure 4: (a) The electromagnetic wave emission and (b) the precursor wave energy averaged in $100 \leq (x-x_{\mathrm{sh}})/(c/\omega_{\mathrm{pe}}) \leq 1500$ for the various positron fractions.
• Figure 5: (a) The downstream energy spectra of positrons for positron fractions of $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.05$ (green), $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.20$ (red), and $N_{0,\rm{e^+}}/N_{0,\rm{e^-}}=0.50$ (blue). (b) The ratio of the number of positrons to the sum of the number of electrons and positrons as a function of $\gamma$ for the various positron fractions.