Dynamics of black hole accretion. Of flux-tubes, plasmoids and transient phenomena
Over the last two decades, general relativistic magnetohydrodynamic simulations (GRMHD) have drawn a robust broad-brush picture of black hole accretion and jet formation in low-luminosity sources. The observations by the Event Horizon Telescope and the GRAVITY interferometer now probe accretion physics with unprecedented detail requiring equally sophisticated dynamical models. I will take this opportunity to review the state-of-the-art in 3D numerical source models and discuss new results obtained with the BlackHoleAccretionCode. To investigate variability and flares, we have analyzed distinct dynamical features like orbiting flux tubes and plasmoid chains that occur in the black hole magnetosphere. Flux tubes escaping from the black hole remain coherent for more than one orbital period and could give rise to flares as currently observed with GRAVITY. The simulations of plasmoids show that trans-relativistic magnetic reconnection is naturally triggered in the accretion flow. Consistent modeling of this process could be key in understanding particle heating and acceleration and I will give an outlook on simulations with the BlackHoleAccretionCode that include plasma resistivity.