Relativistic MHD simulations of extragalactic jets

Abstract

Abstract.We have performed a comprehensive parameter study of the morphology and dynamics of axisymmetric, magnetized, relativistic jets by means of numerical simulations. The simulations have been performed with an upgraded version of the GENESIS code which is based on a second-order accurate finite volume method involving an approximate Riemann solver suitable for relativistic ideal magnetohydrodynamic flows, and a method of lines. Starting from pure hydrodynamic models we consider the effect of a magnetic field of increasing strength (up to β ≡ |b| 2 /2p ≈ 3.3 times the equipartition value) and different topology (purely toroidal or poloidal). We computed several series of models investigating the dependence of the dynamics on the magnetic field in jets of different beam Lorentz factor and adiabatic index. We find that the inclusion of the magnetic field leads to diverse effects which contrary to Newtonian magnetohydrodynamics models do not always scale linearly with the (relative) strength of the magnetic field. The relativistic models show, however, some clear trends. Axisymmetric jets with toroidal magnetic fields produce a cavity which consists of two parts: an inner one surrounding the beam which is compressed by magnetic forces, and an adjacent outer part which is inflated due to the action of the magnetic field. The outer border of the outer part of the cavity is given by the bow-shock where its interaction with the external medium takes place. Toroidal magnetic fields well below equipartition (β = 0.05) combined with a value of the adiabatic index of 4/3 yield extremely smooth jet cavities and stable beams. Prominent nose cones form when jets are confined by toroidal fields and carry a high Poynting flux (σ ≡ |b| 2 /ρ > 0.01 and β ≥ 1). In contrast, none of our models possessing a poloidal field develops such a nose cone. The size of the nose cone is correlated with the propagation speed of the Mach disc (the smaller the speed the larger is the size). If two models differ only by the adiabatic index, jets having smaller adiabatic indices tend to develop smaller nose cones.

Relativistic MHD with adaptive mesh refinement

“…On doing this for our system for the k direction, say, one gets the general relativistic generalization [7] of the seven wave speeds of flat space MHD [26,27,28,29]. These include the entropy wave and two Alfvén waves,…”

Linear wave propagation in relativistic magnetohydrodynamics

“…[4][5][6][7]12 The cold plasma limit is obtained for c g = 0, and has degenerate slow waves. We plot in Fig.…”

“…Expressions for the wave speeds, as obtained for different reference frames in relative motion to each other, have been used as basic ingredients for the current suite of relativistic magnetofluid codes. [4][5][6][7][8][9][10][11][12] Knowledge of the ͑fastest͒ characteristic speeds embedded in the relativistic MHD equations is essential to obtain time-step stability limits for explicit time-stepping strategies, while many modern RMHD codes use more complete information on all linear wave dynamics. This information is incorporated in the eigenvectors of the flux Jacobian, with the fluxes appearing in the governing hyperbolic equations for the conserved variables, typically formulated in a 3 + 1 formulation where a fixed laboratory frame is selected.…”

Chandra Reveals Twin X-ray Jets in the Powerful FR II Radio Galaxy 3C 353

“…In addition, a detailed analysis of the broad-band emission of several other quasar jets supports the synchrotron hypothesis, pointing out an important role of the jet velocity structure (consisting of a fast spine and a slower boundary layer/outer sheath) in shaping the jet high-energy radiation (e.g., Hardcastle 2006, Jester et al 2006. Such jet velocity structure is in fact always seen in numerical modeling of the evolution and propagation of extragalactic relativistic jets (e.g., Aloy et al 1999, Leismann et al 2005, Mizuno et al 2007, since it results inevitably from the non-linear growth of Kelvin-Helmholtz instabilities on the jet surface (see the recent studies by Perucho et al 2007, Hardee 2007, Meliani & Keppens 2007, and references therein). It is not yet established, however, if the jet boundary layers are indeed the places of the enhanced acceleration of high-energy particles.…”