2025A&A...693A.154H

Context. Relativistic jets from active galactic nuclei are observed to be collimated on the parsec scale. When the pressure between the jet and the ambient medium is mismatched, recollimation shocks and rarefaction shocks are formed. Previous numerical simulations have shown that instabilities can destroy the recollimation structure of jets.
Aims. In this study, we aim to study the instabilities of nonequilibrium over-pressured relativistic jets with helical magnetic fields. Especially, we investigate how the magnetic pitch affects the development of instabilities.
Methods. We performed 3D relativistic magnetohydrodynamic simulations for different magnetic pitches, as well as a 2D simulation and a relativistic hydrodynamic simulation which served as comparison groups.
Results. In our simulations, Rayleigh-Taylor instability (RTI) is triggered at the interface between the jet and ambient medium in the recollimation structure of the jet. We find that when the magnetic pitch decreases, the growth of RTI becomes weak but, interestingly, another instability - the current-driven (CD) kink instability - is excited. The excitement of CD kink instability after passing the recollimation shocks can match the explanation of the quasi-periodic oscillations observed in BL Lac qualitatively.