Probing particle acceleration in relativistic jets with simultaneous X-ray polarization and TeV observations

We present results from a joint multi-wavelength campaign on the high-frequency-peaked BL Lac object Mrk 421, combining X-ray polarization observations with IXPE and TeV gamma-ray observations with VERITAS during a high-flux state in December 2023. Simultaneous variability was detected in both bands, with time-resolved X-ray polarization measurements revealing complex polarization degree and angle variations on day timescales. To interpret these observations, we developed a multi-zone emission model based on ensembles of first-principles particle-in-cell (PIC) simulations representing localized magnetic reconnection and magnetized turbulence regions in the relativistic jet. The model statistically compares synthetic light curves of X-ray flux and polarization with the IXPE data, showing that reconnection-dominated configurations more successfully reproduce the observed dynamic polarization signatures than turbulence-dominated scenarios. In addition, the combination of simultaneous X-ray and TeV flux measurements provides an independent constraint on the plasma magnetization in the emission region. This constraint serves as a direct diagnostic of the physical conditions that govern particle acceleration in the jet of Mrk 421, enabling us to assess which acceleration mechanisms are most likely to operate. This work highlights the unique capabilities of first-principles PIC simulations combined with coordinated X-ray and TeV observations to serve as a direct diagnostic of jet plasma conditions and particle acceleration mechanisms.