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 through magnetic reconnection may play a key role in $\gamma$-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. As an important difference with similar works, our accretion discs have an initial dipolar magnetic-field configuration with loops of alternating polarity. We show that current sheets are formed and destroyed rapidly in the turbulent environment of black-hole accretion. Plasmoids are formed from current sheets close to the event horizon, in a region of $\sim 2-15$ 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 $\sim 30-40$ 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*.

1 fig updated, 1 new fig, accepted to MNRAS

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