English

Dependence of Particle Acceleration Efficiency on Shock Velocity in Weakly Magnetized Electron-Ion Shocks

High Energy Astrophysical Phenomena 2026-07-07 v1 Plasma Physics

Abstract

Using unprecedentedly long 2D particle-in-cell simulations, we study electron and ion acceleration in weakly magnetized quasi-parallel shocks, propagating at velocities ranging from transrelativistic to subrelativistic. At a fixed upstream magnetic field strength, low-velocity quasi-parallel shocks are dominated by the Bell instability, whereas high-velocity shocks are dominated by the Weibel instability. Both regimes accelerate ions with similar efficiency, with the Bell-dominated regime exhibiting faster growth in the maximum particle energy. The electron acceleration efficiency is strongly dependent on shock velocity. Weibel-dominated shocks have 15%\sim15\,\% of shock energy in nonthermal electrons, whereas in the Bell-dominated regime we attribute less than 2%\sim2\,\% of shock energy to nonthermal electrons. We discuss applications of our results to the bright X-ray emission from the late-stage afterglows of gamma-ray bursts, the radio emission from fast blue optical transients, and the X-ray variability in microquasars.

Keywords

Cite

@article{arxiv.2607.05778,
  title  = {Dependence of Particle Acceleration Efficiency on Shock Velocity in Weakly Magnetized Electron-Ion Shocks},
  author = {Taiki Jikei and Daniel Groselj and Lorenzo Sironi},
  journal= {arXiv preprint arXiv:2607.05778},
  year   = {2026}
}

Comments

13 pages, 6 figures