Magnetic reconnection and plasmoid formation in three-dimensional accretion flows around black holes

Abstract

Magnetic reconnection is thought to be one of the main energy-dissipation mechanisms fueling energy to the plasma in the vicinity of a black hole. Indeed, plasmoids formed though magnetic reconnection may play a key role in γ-ray, X-ray and near-infrared flares from the black hole at the center of our galaxy, SgrA*. We report the results of three-dimensional general-relativistic ideal and resistive magnetohydrodynamics simulations modelling magnetic reconnection in accretion flows around astrophysical black holes. We show that current sheets are formed and destroyed rapidly in the turbulent environment of black-hole accretion. As this process operates, plasmoids are formed from the current sheets close to the event horizon and in a region of ∼ 2 −15 gravitational radii. We further quantify the magnetic dissipation and the process of energy transfer to the plasmoids, reporting the reconnection rate, the relative current density with respect to the local magnetic field, and the size of the plasmoids. We find that plasmoids gain energy through reconnection and heat up to relativistic temperatures, with the largest ones being sufficiently energetic to leave the black hole near the polar regions. During their evolution, plasmoids are stretched and elongated, becoming disrupted when the shear is sufficiently large, although some plasmoids survive as well-distinguished structures at distances of ∼ 30 − 40 gravitational radii from the black hole. Finally, we find that in some cases the plasmoids acquire a super-Keplerian azimuthal velocity, as suggested by recent observations of flares from Sgr A*.

First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole

“…It may signal the presence of extended, slowly varying structure that is resolved out by EHT, or it may signal that future models need to incorporate collisionless effects (potentially modeled as viscosity and conductivity) or a more sophisticated treatment of electron thermodynamics including cooling. In addition, different initial geometries and polarities of the magnetic field could lead to more slowly varying structures (Nathanail et al 2021). If, when combined, these effects were to reduce M 3 by 30%, then many MAD models would be consistent with the data.…”

“…For the 50% change in resolution considered in the comparison shown in the preceding appendix (between KHARMA and BHAC simulations) we find no evidence for systematic changes in M 3 with resolution. This is not a large range in resolution, however, and much higher resolution simulations (Ripperda et al 2020(Ripperda et al , 2022Nathanail et al 2021) show the emergence of qualitatively new structures (plasmoids) in current sheets that could affect 230 GHz variability. A deeper study of the resolution dependence of variability is clearly warranted but is beyond the scope of this paper.…”

GRMHD simulations of accreting neutron stars I: nonrotating dipoles

“…Although the magnetisation of these magnetic islands is larger than that of the rest of plasma, it is nevertheless modest and of order unity. Nevertheless, these structures share many of the properties of the plasmoids found in accreting supermassive black holes (Nathanail et al 2020(Nathanail et al , 2021. Second, because the stellar magnetic field around the torus is weaker than the one initially seeded in the torus, the latter expands due to magnetic buoyancy as the simulation proceeds, giving rise to a lowdensity magnetised plasma, i.e., a "corona" (see top rows of Figs.…”

GRMHD Simulations and Modeling for Jet Formation and Acceleration Region in AGNs

“…In general, different poloidal magnetic field configurations result in different magnetic flux accretions onto the black hole horizon and jet launching properties (e.g., [76,77,112,[122][123][124]). In particular, the configuration of multiple poloidal magnetic loops with different polarities leads to intermittent, non-stationary jet formation (e.g., [125,126]). In same-polarity multiple poloidal magnetic loop cases, same-polarity loops quickly reconnect to form a large loop with a resulting flow similar to the single magnetic loop case.…”