Black hole flares: ejection of accreted magnetic flux through 3D plasmoid-mediated reconnection

TL;DR

This study uses high-resolution 3D simulations to reveal how magnetic reconnection in black hole magnetospheres leads to flux ejection and flare phenomena, aligning with observations of objects like Sgr A* and M87.

Contribution

First 3D high-resolution simulation capturing plasmoid-mediated reconnection in a magnetically arrested disk around a black hole.

Findings

Magnetic flux escapes through reconnection at a universal rate.

Flux bundles form hot spots consistent with Sgr A* observations.

Reconnection energizes plasma to explain high-energy flares from black holes.

Abstract

Magnetic reconnection can power bright, rapid flares originating from the inner magnetosphere of accreting black holes. We conduct extremely high resolution ($5376\times2304\times2304$ cells) general-relativistic magnetohydrodynamics simulations, capturing plasmoid-mediated reconnection in a 3D magnetically arrested disk for the first time. We show that an equatorial, plasmoid-unstable current sheet forms in a transient, non-axisymmetric, low-density magnetosphere within the inner few Schwarzschild radii. Magnetic flux bundles escape from the event horizon through reconnection at the universal plasmoid-mediated rate in this current sheet. The reconnection feeds on the highly-magnetized plasma in the jets and heats the plasma that ends up trapped in flux bundles to temperatures proportional to the jet's magnetization. The escaped flux bundles can complete a full orbit as low-density hot spots, consistent with Sgr A$^{*}$ observations by the GRAVITY interferometer. Reconnection near the horizon produces sufficiently energetic plasma to explain flares from accreting black holes, such as the TeV emission observed from M87. The drop in mass accretion rate during the flare, and the resulting low-density magnetosphere make it easier for very high energy photons produced by reconnection-accelerated particles to escape. The extreme resolution results in a converged plasmoid-mediated reconnection rate that directly determines the timescales and properties of the flare.