Probing particle acceleration mechanisms with multi-wavelength polarimetry

Blazars are among the most powerful particle accelerators in the Universe. In particular, high-synchrotron-peaked blazars (HSPs), whose synchrotron component peaks in the X-ray bands, can accelerate nonthermal electrons to above TeV energies, and they are potential cosmic ray accelerators. However, the particle accelerator mechanisms are not well understood. Polarimetry can be powerful diagnostics for particle acceleration as different mechanisms involve very different magnetic field evolution. The recent IXPE discoveries that the optical and X-ray polarization signatures are drastically different in HSPs suggest that nonthermal electrons of different energies observe very different magnetic field structure, which can help to distinguish particle acceleration mechanisms. This talk will present combined particle-in-cell and radiation transfer simulations for HSPs under the magnetic reconnection and turbulence scenarios. The simulation results show that both mechanisms can lead to similar energy stratification effects that lead to higher X-ray polarization degree than the optical counterpart. Additionally, these simulations provide alternative scenarios for the observed X-ray polarization angle swings, but with distinct predictions from the shock model in the IXPE paper, which can be examined with future multi-wavelength campaigns.