Particle Acceleration by Pickup Process Upstream of Relativistic Shocks

Abstract

Particle acceleration at magnetized purely perpendicular relativistic shocks in electron-ion plasmas are studied by means of two-dimensional particle-in-cell simulations. Magnetized shocks with the upstream bulk Lorentz factor γ 1 1 are known to emit intense electromagnetic waves from the shock front, which induce electrostatic plasma waves (wakefield) and transverse filamentary structures in the upstream region via the stimulated/induced Raman scattering and the filamentation instability, respectively. The wakefield and filaments inject a fraction of incoming particles into a particle acceleration process, in which particles are once decoupled from the upstream bulk flow by the wakefield, and are piked up again by the flow. The picked-up particles are accelerated by the motional electric field. The maximum attainable Lorentz factor is estimated as γ max,e ∼ αγ 3 1 for electrons and γ max,i ∼ (1 + m e γ 1 /m i )γ 2 1 for ions, where α ∼ 10 is determined from our simulation results. α can increase up to γ 1 for weakly magnetized shock if γ 1 is sufficiently large. This result indicates that highly relativistic astrophysical shocks such as external shocks of gamma-ray bursts can be an efficient particle accelerator.

Reverse shock forming condition for magnetized relativistic outflows: reconciling theories and simulations

“…This is partly supported by the particle acceleration mechanism. For ultra-relativistic shocks, numerical simulations suggested that particles can only be accelerated efficiently via the diffusive shock acceleration when 𝜎 10 −3 (Sironi & Spitkovsky 2009Sironi et al 2013), but precursor waves generated by the synchrotron maser instability may operate as an alternative acceleration mechanism up to 𝜎 ∼ 0.1 − 1 (Gallant et al 1992;Iwamoto et al 2017Iwamoto et al , 2018Plotnikov & Sironi 2019;Sironi et al 2021;Iwamoto et al 2021). It is less clear whether these conclusions hold for mildly relativistic shocks (Sironi & Spitkovsky 2011;Ligorini et al 2021), which could be important for a transrelativistic RS.…”