Most of the TeV high-frequency-peaked BL Lacs (HBLs) show stationary orslow motions of their VLBI radio-knots, in stark contrast with their fast variability. This problem, known as "the bulk Lorentz factor crisis," can be resolved if we consider that these strings of knots are successive recollimation shocks in their jets. Successive shocks predict that a unique pattern of the non-thermal emission variability should appear after each strong flare. Focusing on the well known HBL Mrk 421 and using the 13-year long X-ray dataset from the Swift X-ray Telescope, we have found evidence of such a distinct pattern with a statistical significance of more than 3 standard deviations compared to variability from stochastic fluctuations. I present how this pattern can be used in an innovative way to unveil the physical properties of the Mrk 421 jet, such as the apparent flow speed, the size of jet perturbations, and the jet beaming parameters. Finally, I discuss the possibility of multiple-shocks to re-accelerate particles in jets, and the ongoing works extending this study with MHD simulations and for other wavelengths.
Speaker: Olivier Hervet, UC Santa Cruz
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