Evolution of accretion discs around a kerr black hole using extended magnetohydrodynamics

Abstract

Black holes accreting well below the Eddington rate are believed to have geometrically thick, optically thin, rotationally supported accretion discs in which the Coulomb mean free path is large compared to GM/c 2 . In such an environment, the disc evolution may differ significantly from ideal magnetohydrodynamic predictions. We present non-ideal global axisymmetric simulations of geometrically thick discs around a rotating black hole. The simulations are carried out using a new code grim, which evolves a covariant extended magnetohydrodynamics model derived by treating non-ideal effects as a perturbation of ideal magnetohydrodynamics. Non-ideal effects are modeled through heat conduction along magnetic field lines, and a difference between the pressure parallel and perpendicular to the field lines. The model relies on an effective collisionality in the disc from wave-particle scattering and velocity-space (mirror and firehose) instabilities. We find that the pressure anisotropy grows to match the magnetic pressure, at which point it saturates due to the mirror instability. The pressure anisotropy produces outward angular momentum transport with a magnitude comparable to that of MHD turbulence in the disc, and a significant increase in the temperature in the wall of the jet. We also find that, at least in our axisymmetric simulations, conduction has a small effect on the disc evolution because (1) the heat flux is constrained to be parallel to the field and the field is close to perpendicular to temperature gradients, and (2) the heat flux is choked by an increase in effective collisionality associated with the mirror instability.

The disc-jet symbiosis emerges: modelling the emission of Sagittarius A* with electron thermodynamics

“…This is because the field lines in our simulation are primarily toroidal while the temperature gradients are primarily poloidal. Foucart et al (2016) came to a similar conclusion for the effect of ion conduction on the fluid dynamics when including it in the total fluid stress-energy tensor. The small effect of electron conduction in our model suggests that future modelling of electron thermodynamics can probably neglect electron conduction, greatly simplifying the numerics and reducing the computational expense by a factor of ∼ 4.…”

Radiative, two-temperature simulations of low-luminosity black hole accretion flows in general relativity

“…We ignore, however, anisotropic heat conduction along magnetic field lines. The effect of this mechanism on optically thin accretion flows has recently been studied by Ressler et al (2015), Chandra et al (2015), and Foucart et al (2016). Ressler et al (2015), in particular, have shown that the anisotropic heat conduction contributes to at most 20% of the isotropic heat flux.…”

Observational Signatures of Mass-loading in Jets Launched by Rotating Black Holes

“…This ratio is a free parameter that is poorly constrained both by theory and observations. We choose values consistent with the findings of recent, sophisticated models of the electron thermodynamics in collisionless accretion flows (Ressler et al 2015;Foucart et al 2016;Saḑowski et al 2017), which show that the electron temperature is comparable to the proton temperature in highly magnetized regions of the flow. Modeling the electron physics in accretion disks and jets remains an active area of research, which will hopefully be informed further by upcoming observations with the EHT.…”