Top: 15.3 GHz VLBI map of Mrk 421 taken Jan. 14th 2011 showing 4 radio knots downstream the core (adapted from Lico et al. 2012). Bottom: Lightcurve resulting of stacked flares fitted by a multi-Gaussian function. A variability pattern is identified in agreement with a relativistic perturbation crossing the radio knots (Hervet et al. 2019).The multiple quasi-stationary radio knots observed in HBL sources can be interpreted as recollimation shocks accelerating particles. This approach has the advantage to resolve a famous scientific issue in blazars, "the Lorentz factor crisis". It relates to the apparent contradiction between the large jet Lorentz factors deduced from high energy emission and fast variability, and the observations of quasi-stationary knots in some of these sources.
One prediction of the recollimation shock scenario is that a distinct variability pattern should appear after each powerful flare, attesting of a perturbation passage trough the knotty jet structure.
As a first study of this effect, I focused my attention on the most luminous HBL in the sky showing multiple stationary knots, Mrk 421. I developed a specific method to analyze lightcurves in order to test this scenario, applying it to an X-ray dataset of 13 years of observation by the space telescope Swift-XRT.
In 2018 I was granted 80ks of observing time by the Swift collaboration as PI (Swift Guest Investigator program, Cycle 14). This campaign was triggered from a historical flare in Jan. 2018; three distinct post-flare events were observed in the periods expected by this scenario.
From numerous flares observed in the 13 years dataset, I observed a recurring variability pattern consistent with perturbations moving at apparent speed of 45 c crossing the successive knots (see main Figure). The probability of this pattern being associated with stochastic fluctuations is rejected at more than 3 standard deviations.
In addition of highlighting the role of stationary radio knots as powerful particle accelerators, this method allows an estimation of the speed and size of ejectas without any simultaneous jet imaging.