Probing an X-Ray Flare Pattern in Mrk 421 Induced by Multiple Stationary Shocks: A Solution to the Bulk Lorentz Factor Crisis

The common observations of multiple radio VLBI stationary knots in high-frequency-peaked BL Lacs (HBLs) can be interpreted as multiple recollimation shocks accelerating particles along jets. This approach can resolve the so-called "bulk Lorentz factor crisis" of sources with a high Lorentz factor deduced from maximum γ-γ opacity and fast variability and apparently inconsistent slow/stationary radio knots. It also suggests that a unique pattern of the nonthermal emission variability should appear after each strong flare. Taking advantage of the 13 yr of observation of the HBL Mrk 421 by the X-ray Telescope on the Neil Gehrels Swift Observatory (Swift-XRT), we probe for such an intrinsic variability pattern. Its significance is then statistically estimated via comparisons with numerous similar simulated light curves. A suggested variability pattern is identified, consistent with a main flare emission zone located in the most upstream 15.3 GHz radio knot at 0.38 mas from the core. Subsequent flux excesses in the light curve are consistent with a perturbation crossing all of the downstream radio knots with a constant apparent speed of 45c. The significance of the observed variability pattern not arising from stochastic processes is found above three standard deviations, opening a promising path for further investigations in other blazars and with other energy bands. In addition to highlighting the role of stationary radio knots as high-energy particle accelerators in jets, the developed method allows estimates of the apparent speed and size of a jet perturbation without the need to directly observe any motion in jets.