Interface instabilities in hydrodynamic relativistic jets

TL;DR

This study investigates how interface instabilities like RTI and RMI influence the evolution, turbulence, and mixing in relativistic jets through 3D hydrodynamic simulations, with implications for various astrophysical phenomena.

Contribution

It provides the first detailed relativistic simulation analysis of RTI and RMI in non-magnetized jets, revealing their roles in jet dynamics and mixing processes.

Findings

RTI growth matches relativistic predictions in jet collimation zones.

RMI becomes significant downstream of collimation shocks.

Instabilities induce turbulence and jet-ambient matter mixing.

Abstract

Both the dynamics and the observational properties of relativistic jets are determined by their interaction with the ambient medium. A crucial role is played by the contact discontinuity at the jet boundary, which in the presence of jet collimation may become subject to Rayleigh-Taylor instability (RTI) and Richtmyer-Meshkov instability (RMI). Here, we study the evolution of these instabilities in non-magnetized relativistic jets using special relativistic three-dimensional hydrodynamic simulations. We show that the growth of initial perturbations is consistent with relativistic RTI operating in the jet collimation region. The contribution of RMI becomes important downstream from the collimation shock in agreement with theoretical expectations. Both instabilities reach non-linear scales above the shock convergence point and trigger strong turbulence, mixing jet with ambient matter. We devise an analytic solution for the mixing rate and show that it is sensitive to the external density gradients. Our results may be applied to different types of astrophysical objects. In particular, different contribution of interface instabilities is a natural explanation for the observed dichotomy between FR-I and FR-II radiogalaxies. The rapid slow-down in the jet of M87 is consistent with baryon entrainment from the circumnuclear matter with the observed density distribution. In microquasars, baryon loading triggered by interface instabilities is a probable reason for the low observed Lorentz factors. We show that the observed variability in gamma-ray bursts cannot come from mixing driven by interface instabilities and likely originates from the engine, suggesting the presence of magnetic fields in the jet.