Magnetic acceleration of relativistic active galactic nucleus jets

Abstract

We present numerical simulations of axisymmetric, magnetically driven relativistic jets. Our special‐relativistic, ideal‐magnetohydrodynamics numerical scheme is specifically designed to optimize accuracy and resolution and to minimize numerical dissipation. In addition, we implement a grid‐extension method that reduces the computation time by up to three orders of magnitude and makes it possible to follow the flow up to six decades in spatial scale. To eliminate the dissipative effects induced by a free boundary with an ambient medium we assume that the flow is confined by a rigid wall of a prescribed shape, which we take to be z∝ra (in cylindrical coordinates, with a ranging from 1 to 3). We also prescribe, through the rotation profile at the inlet boundary, the injected poloidal current distribution: we explore cases where the return current flows either within the volume of the jet or on the outer boundary. The outflows are initially cold, sub‐Alfvénic and Poynting flux‐dominated, with a total‐to‐rest‐mass energy flux ratio μ∼ 15. We find that in all cases they converge to a steady state characterized by a spatially extended acceleration region. The acceleration process is very efficient: on the outermost scale of the simulation as much as ∼ 77 per cent of the Poynting flux has been converted into kinetic energy flux, and the terminal Lorentz factor approaches its maximum possible value (Γ∞≃μ). We also find a high collimation efficiency: all our simulated jets (including the limiting case of an unconfined flow) develop a cylindrical core. We argue that this could be the rule for current‐carrying outflows that start with a low initial Lorentz factor (Γ0∼ 1). Our conclusions on the high acceleration and collimation efficiencies are not sensitive to the particular shape of the confining boundary or to the details of the injected current distribution, and they are qualitatively consistent with the semi‐analytic self‐similar solutions derived by Vlahakis and Königl. We apply our results to the interpretation of relativistic jets in active galactic nuclei: we argue that they naturally account for the spatially extended accelerations inferred in these sources (Γ∞≳ 10 attained on radial scales R≳ 1017 cm) and are consistent with the transition to the matter‐dominated regime occurring already at R≳ 1016 cm.

Modelling the multi-wavelength emission of flat-spectrum radio quasar 3C 279

“…This assumption results to a pure toroidal magnetic field distribution in the jet, while a polodial magnetic distribution is adopted in an inhomogeneous jet model for the rapid variability of TeV blazars (Boutelier et al 2008). We note that a domi- nant toroidal magnetic field in jet are predicted by theory (Lyutikov et al 2005) and magnetohydrodynamic (MHD) simulations (McKinney 2006;Komissarov et al 2007), and are also supported by a parsec-scale polarization observation (O'Sullivan & Gabuzda 2009;Kharb et al 2009).…”

On the magnetization of BL Lac jets

“…The jet power, originally carried by an almost pure electromagnetic beam (with magnetization parameter σ ≡ PB/P kin ≫ 1), is progressively used to accelerate matter, until a substantial equipartition between the magnetic and the kinetic energy fluxes (σ ≈ 1) is established (e.g., Komissarov et al 2007, Tchekhovskoy et al 2009, Vlahakis 2015. Dissipation of part of the kinetic (through shocks) and/or magnetic (through reconnection) power leads to the acceleration of particles up to ultra-relativistic energies, producing the nonthermal emission we observe from the jets of blazars, i.e.…”

Hadronic jet models today

“…Considering the minimal cross-sectional radius of a jet to be R ∼ 10 15 cm, which for θ j ∼ 1/Γ corresponds with a distance 100 gravitational radii of the 10 9 M BH -required to be at least of this order to accelerate the jet up to Γ ∼ 10 ( Komissarov et al 2007) -we can find using Eqs. (3.10) and (3.11) that (4.4) This indicates that even for such extreme parameters as f ∼ 1 and σ ∼ 1 the efficiency is too small to explain γ-ray luminosities L γ ∼ 10 44 erg s −1 , unless one assumes very hard (q p < 1) proton injection spectra and adopts significantly larger total jet power.…”

“…This leads to a formation of Poynting flux dominated outflows. Those in turn can at some point be converted to the matter dominated jets (Komissarov et al 2007;Tchekhovskoy et al 2009;Lyubarsky 2010;Komissarov 2010), with a terminal Lorentz factor Γ ∼ P j /Ṁc 2 where P j is the rate of energy extraction from rotating BH and/or accretion disk andṀ is the mass flux. Depending on whether a jet is launched in the BH magnetosphere or by the accretion disk, the mass flux is expected to be dominated by electron/positron pairs or by protons.…”