Jet launching in resistive GR-MHD black hole - accretion disk systems

TL;DR

This study uses resistive GR-MHD simulations to explore how magnetic diffusivity and black hole spin influence relativistic jet launching from accretion disks, revealing the dominant role of disk winds over central electromagnetic energy flux.

Contribution

It extends resistive GR-MHD modeling to black hole accretion disks, analyzing the effects of magnetic diffusivity and black hole spin on jet launching mechanisms.

Findings

Magnetic diffusivity reduces accretion and ejection efficiency.

Disk winds are mildly relativistic and dominate energy flux.

Black hole spin suppresses accretion via frame-dragging effects.

Abstract

We investigate the launching mechanism of relativistic jets from black hole sources, in particular the strong winds from the surrounding accretion disk. Numerical investigations of the disk wind launching - the simulation of the accretion-ejection transition - have so far almost only been done for non-relativistic systems. From these simulations we know that resistivity, or magnetic diffusivity, plays an important role for the launching process. Here, we extend this treatment to general relativistic magnetohydrodynamics (GR-MHD) applying the resistive GR-MHD code rHARM. Our model setup considers a thin accretion disk threaded by a large-scale open magnetic field. We run a series of simulations with different Kerr parameter, field strength and diffusivity level. Indeed we find strong disk winds with, however, mildly relativistic speed, the latter most probably due to our limited computational domain. Further, we find that magnetic diffusivity lowers the efficiency of accretion and ejection, as it weakens the efficiency of the magnetic lever arm of the disk wind. As major driving force of the disk wind we disentangle the toroidal magnetic field pressure gradient, however,magneto-centrifugal driving may also contribute. Black hole rotation in our simulations suppresses the accretion rate due to an enhanced toroidal magnetic field pressure that seems to be induced by frame-dragging. Comparing the energy fluxes from the Blandford-Znajek-driven central spine and the surrounding disk wind, we find that the total electromagnetic energy flux is dominated by the total matter energy flux of the disk wind (by a factor 20). The kinetic energy flux of the matter outflow is comparatively small and comparable to the Blandford-Znajek electromagnetic energy flux.