Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc

TL;DR

This paper demonstrates that magnetic reconnection events in a magnetically arrested disc around Sgr A* can explain the observed near-infrared flares, including their rotation and polarization characteristics.

Contribution

The study shows that GRMHD simulations of magnetically arrested discs can produce flare phenomena consistent with observations, highlighting magnetic reconnection as a key process.

Findings

Reconnection-driven eruptions expel magnetically dominated plasma.

Simulated flares match observed durations and amplitudes.

Rotation of emission region causes polarization angle changes.

Abstract

Large-amplitude Sgr A* near-infrared flares result from energy injection into electrons near the black hole event horizon. Astrometry data show continuous rotation of the emission region during bright flares, and corresponding rotation of the linear polarization angle. One broad class of physical flare models invokes magnetic reconnection. Here we show that such a scenario can arise in a general relativistic magnetohydrodynamic simulation of a magnetically arrested disc. Saturation of magnetic flux triggers eruption events, where magnetically dominated plasma is expelled from near the horizon and forms a rotating, spiral structure. Dissipation occurs via reconnection at the interface of the magnetically dominated plasma and surrounding fluid. This dissipation is associated with large increases in near-infrared emission in models of Sgr A*, with durations and amplitudes consistent with the observed flares. Such events occur at roughly the timescale to re-accumulate the magnetic flux from the inner accretion disc, 10h for Sgr A*. We study near-infrared observables from one sample event to show that the emission morphology tracks the boundary of the magnetically dominated region. As the region rotates around the black hole, the near-infrared centroid and linear polarization angle both undergo continuous rotation, similar to the behavior seen in Sgr A* flares.