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The common observations of multiple radio VLBI stationary knots inhigh-frequency-peaked BL Lacs (HBLs) can be interpreted as multiplerecollimation shocks accelerating particles along jets. This approach canresolve the so-called "bulk Lorentz factor crisis" of sources with high Lorentzfactor, deduced from maximum gamma-gamma opacity and fast variability, andapparently inconsistent slow/stationary radio knots. It also suggests that aunique pattern of the non-thermal emission variability should appear after eachstrong flare. Taking advantage of the 13 years of observation of the HBL Mrk421 by the X-ray Telescope on the Neil Gehrels Swift Observatory Swift-XRT, weprobe for such an intrinsic variability pattern. Its significance is thenstatistically estimated via comparisons with numerous similar simulatedlightcurves. A suggested variability pattern is identified, consistent with amain flare emission zone located in the most upstream 15.3 GHz radio knot at0.38 mas from the core. Subsequent flux excesses in the lightcurve areconsistent with a perturbation crossing all the downstream radio knots with aconstant apparent speed of 45 c. The significance of the observed variabilitypattern not arising from stochastic processes is found above 3 standarddeviations, opening a promising path for further investigations in otherblazars and with other energy bands. In addition to highlight the role ofstationary radio knots as high-energy particle accelerators in jets, thedeveloped method allows estimates of the apparent speed and size of a jetperturbation without the need to directly observe any motion in jets.

Probing an X-ray flare pattern in Mrk 421 induced by multiple stationary shocks: a solution to the bulk Lorentz factor crisis