Energetic particle beams in quasars and active galactic nuclei

Abstract

New developments of a two-flow model for extragalactic jets are presented. We study the possibility of ascribing Very Long Baseline Interferometry (VLBI) jet features and superluminal motion to beams of relativistic particles streaming through an ambient plasma such as a wind from the accretion disc. It is shown that stability of the beams relative to the excitation of Langmuir and Alfvénic waves can be ensured in the case of electron–positron beams with bulk Lorentz factor γ smaller than 43 (the square root of proton to electron mass ratio) as long as the magnetic field parallel to the beams and jets is larger than a critical value |${B}_\text{c}=3.2\times{10}^{-3}\,n_\text{p}^{1/2},\,\text{where}\,{n}_\text{p}$| is the density of the ambient plasma. Conversely, electron–positron beams with higher bulk Lorentz factor, and electron–proton beams are rapidly destroyed. Therefore we propose that electron–positron beams with |$\gamma \leq 43$| are responsible for VLBI features. Two main models for VLBI structures are then expected according to the relative values of parallel magnetic field and ambient plasma density close to the central engine, corresponding to two different regimes for the beams, namely propagation when |$B\gt{B}_\text{c}$| and dissolution when |$B\lt{B}_\text{c}.$|