PIC simulations of particle acceleration at relativistic magnetized shocks

The efficiency of particle acceleration at shock waves in relativistic, magnetized astrophysical outflows is a debated topic with far-reaching implications. Most of previous numerical studies based on fully PIC simulations consider laminar in-flow conditions, i.e., nonturbulent, homogeneous background plasma of uniform magnetization. In this talk, based on a recent study, I will show how the presence of a well-developed turbulence upstream of a fast shock may change the picture. In particular, we carried out PIC simulations of a mildly relativistic magnetized pair shock (Lorentz factor γsh ≃ 2.7, magnetization level σ ≃ 0.01), and we found that strong turbulence can revive particle acceleration in a superluminal configuration that otherwise prohibits it. By the analysis of tracked particles we investigated the acceleration process and could conclude that, depending on the initial plasma temperature and magnetization, shock-drift or diffusive-type acceleration governs particle energization, producing power-law spectra dN/dγ ∝ γ^−s with s ≈ 2.5–3.5. At larger magnetization levels, stochastic acceleration within the preshock turbulence becomes competitive and can even take over shock acceleration.