Physics of Weakly Magnetized Transrelativistic Shocks

We discuss the physics of transrelativistic shocks, motivated by late-time observation of gamma-ray burst (GRB) afterglows, recent discovery of fast blue optical transients (FBOTs), and the recent detection of ~100 TeV gamma rays. We use 2D particle-in-cell simulations to probe magnetic field amplification and particle acceleration at transrelativistic shocks in weakly magnetized electron-ion plasmas. We show that there is a competition between two plasma instabilities: the Bell instability and the Weibel instability. They both can amplify the magnetic field to a much higher level than the ambient field. The Bell instability dominates at relatively large magnetization (or low Alfvenic Mach number) and leads to fast ion acceleration. The Weibel instability prevails at lower magnetization (or high Alfvenic Mach number) and accelerates both electrons and ions, though at a slower rate.