Top: Quasi-stationary knots observed in the jet of M 87 (Asada et al. 2014). Bottom: 2D simulation of the pressure profile of a two-flows jet where the kinetic energy of the outer jet widely dominates the one of the inner jet (Hervet et al. 2017). The shock structure of this simulation shows multiple similarities with the radio structure of M 87.The observation of successive stationary knots in numerous jets (see main Figure , Top) brought me into the study of recollimation shocks. Indeed, a supersonic (or super-magnetosonic) jet with a strong pressure disequilibrium with the outer medium naturally produces stationary recollimation shocks. These are for example typical of rocket or jet engines, often designated as diamond chocks (from their shape), or Mach disks.
When considering an isothermal jet approximation with constant flow speed, an interesting feature of these shocks is that their relative distance from one to another is directly proportional to the jet radius. By the study of a sample of jets with multiple stationary knots I checked the existence of this relation of proportionality in most of the studied sources.
It thus appears that radio knots display positions and sizes as expected for recollimation shocks.
This observation makes unlikely multiple other interpretations regarding the nature of these radio knots.
In the frame of this study I have built a collaboration with Dr. Zakaria Meliani from the Paris Observatory, specialist in jet MHD simulations. Our goal is to reproduce the various jet structures and kinematics observed in radio VLBI by relativistic MHD simulations (see main Figure, Bottom).
Our preliminary studies quickly concluded that the diversity of observed jets can only be satisfyingly reproduced by two-flows simulations. In this context, the jets have an inner component with a strongly relativistic flow, which is responsible of most of the high energy emission, and characterized by the observed VLBI radio knots. This inner jet is embedded in an outer jet, which contributes in most of the radio emission and displays a lower Lorentz factor.
The two-flows scenario is supported by theoretical plasma physics (Sol et al. 1989), theoretical jet launching mechanisms (Blandfor & Znajeck 1977, Blandford & Payne 1982), disk-jet connection simulations (Ferreira et al. 2006), and probably mainly from direct radio VLBI imaging (Mertens et al. 2016).
This study allowed us to associate the kinematic classification from my 2016 paper with specific simulated two-flows jets. From this association I deduced that radio VLBI jet structures strongly depend of the kinetic power ratio between inner and outer jets. This brings a new look on our AGN understanding, where sources are usually classified only following the total power output of their jets.
I consequently proposed the emergence of a new paradigm in the jetted AGN classification where one can now link in a consistent scheme multiwavelength emission, radio VLBI structure and kinematics, recollimation shocks, and two-flows jets.