Plasmoid formation in global GRMHD simulations and AGN flares

Abstract

One of the main dissipation processes acting on all scales in relativistic jets is thought to be governed by magnetic reconnection. Such dissipation processes have been studied in idealized environments, such as reconnection layers, which evolve in merging islands and lead to the production of ‘plasmoids’, ultimately resulting in efficient particle acceleration. In accretion flows on to black holes, reconnection layers can be developed and destroyed rapidly during the turbulent evolution of the flow. We present a series of two-dimensional general-relativistic magnetohydrodynamic simulations of tori accreting on to rotating black holes focusing our attention on the formation and evolution of current sheets. Initially, the tori are endowed with a poloidal magnetic field having a multiloop structure along the radial direction and with an alternating polarity. During reconnection processes, plasmoids and plasmoid chains are developed leading to a flaring activity and hence to a variable electromagnetic luminosity. We describe the methods developed to track automatically the plasmoids that are generated and ejected during the simulation, contrasting the behaviour of multiloop initial data with that encountered in typical simulations of accreting black holes having initial dipolar field composed of one loop only. Finally, we discuss the implications that our results have on the variability to be expected in accreting supermassive black holes.

The Jet–disk Boundary Layer in Black Hole Accretion

“…The current-vortex sheet can be subject to Kelvin-Helmholtz instability (KHI) as well as the plasmoid instability (Loureiro et al 2007). High-resolution axisymmetric models of black hole accretion flows (Nathanail et al 2020, Ripperda et al 2020) see evidence for plasmoid instability at the jet-disk boundary, but we do not, perhaps due to inadequate resolution. We therefore focus on KHI.…”

“…Sironi et al (2021) performed 2D particle-in-cell simulations of the shear layer between a relativistic, magnetically dominated electron-positron jet and a weakly magnetized ion-electron plasma and showed that the nonlinear evolution of KHIs leads to magnetic reconnection, which can in turn drive particle acceleration. The formation of magnetic islands at the jet-disk boundary (see, e.g., Nathanail et al 2020;Ripperda et al 2020) can also lead to particle acceleration; this process has been extensively investigated in kinetic simulations of current sheets.…”

Polarimetric Properties of Event Horizon Telescope Targets from ALMA

“…Asada et al 2002;Gómez et al 2008;Gabuzda et al 2014). Systematic changes in the signs of these gradients, leading to RM sign reversals in unresolved measurements, can be explained with a number of models, including the magnetic "tower" model (Lynden-Bell 1996; Contopoulos & Kazanas 1998;Lico et al 2017), or the "striped" jet model (Parfrey et al 2015;Mahlmann et al 2020;Nathanail et al 2020). Nevertheless, it remains difficult to explain the rapid fluctuations observed in 2017 April with these models.…”

Computational general relativistic force-free electrodynamics

“…The longest variability timescales in the observed TeV emission, for example, in M87, have recently been linked to recurring periods of efficient Blandford/Znajek (Blandford & Znajek 1977) type outflows induced by the accretion of magnetic flux tubes (Parfrey et al 2015;Mahlmann et al 2020). Reconnection and plasmoid formation in BH accretion processes are likely to act on much shorter timescales (studied in the ideal limit by, e.g., Nathanail et al 2020) and involve relevant physical nonideal electric fields (analyzed in the resistive limit by, e.g., Ripperda et al 2020). A large array of work makes use of numerical laboratories set up in (GR)FFE in order to simulate the most extreme environments of the universe while constantly breaking their own limits.…”