General relativistic magnetohydrodynamic simulations of the jet formation and large-scale propagation from black hole accretion systems

Abstract

The formation and large-scale propagation of Poynting-dominated jets produced by accreting, rapidly rotating black hole systems are studied by numerically integrating the general relativistic magnetohydrodynamic equations of motion to follow the self-consistent interaction between accretion disks and black holes. This study extends previous similar work by studying jets till $t\approx 10^4GM/c^3$ out to $r\approx 10^4GM/c^2$, by which the jet is super- fast magnetosonic and moves at a lab-frame bulk Lorentz factor of $\Gamma\sim 10$ with a maximum terminal Lorentz factor of $\Gamma_\infty\lesssim 10^3$. The radial structure of the Poynting-dominated jet is piece-wise self-similar, and fits to flow quantities along the field line are provided. Beyond the \alf surface at $r\sim 10$--$100GM/c^2$, the jet becomes marginally unstable to (at least) current-driven instabilities. Such instabilities drive shocks in the jet that limit the efficiency of magnetic acceleration and collimation. These instabilities also induce jet substructure with $3\lesssim\Gamma\lesssim 15$. The jet is shown to only marginally satisfy the necessary and sufficient conditions for kink instability, so this may explain how astrophysical jets can extend to large distances without completely disrupting. At large distance, the jet angular structure is Gaussian-like (or uniform within the core with sharp exponential wings) with a half-opening angle of $\approx 5^\circ$ and there is an extended component out to $\approx 27^\circ$. Unlike in some hydrodynamic simulations, the environment is found to play a negligible role in jet structure, acceleration, and collimation as long as the ambient pressure of the surrounding medium is small compared to the magnetic pressure in the jet.