Flux eruption events drive angular momentum transport in magnetically arrested accretion flows

TL;DR

This study uses high-resolution GRMHD simulations to analyze how magnetic flux eruptions in MAD accretion flows influence angular momentum transport and wind launching around non-spinning black holes.

Contribution

It demonstrates that flux eruptions in MADs cause strong vertical angular momentum transport and reveal the highly transient, non-axisymmetric nature of MAD states, contrasting with SANE accretion.

Findings

Flux eruptions drive strong winds with 10% of accretion power.

MAD states are highly transient and often switch to SANE-like states.

Angular momentum transport differs: vertical in MADs, equatorial in SANEs.

Abstract

We evolve two high-resolution general relativistic magnetohydrodynamic (GRMHD) simulations of advection-dominated accretion flows around non-spinning black holes (BHs), each over a duration $\sim 3\times 10^5\,GM_{\rm BH}/c^3$. One model captures the evolution of a weakly magnetized (SANE) disk and the other a magnetically arrested disk (MAD). Magnetic flux eruptions in the MAD model push out gas from the disk and launch strong winds with outflow efficiencies at times reaching $10\%$ of the incoming accretion power. Despite the substantial power in these winds, average mass outflow rates remain small out to a radius $\sim100\,GM_{\rm BH}/c^2$, only reaching $\sim 60-80\%$ of the horizon accretion rate. The average outward angular momentum transport is primarily radial in both modes of accretion, but with a clear distinction: magnetic flux eruption-driven disk winds cause a strong vertical flow of angular momentum in the MAD model, while for the SANE model, the magnetorotational instability (MRI) moves angular momentum mostly equatorially through the disk. Further, we find that the MAD state is highly transitory and non-axisymmetric, with the accretion mode often changing to a SANE-like state following an eruption before reattaining magnetic flux saturation with time. The Reynolds stress changes direction during such transitions, with the MAD (SANE) state showing an inward (outward) stress, possibly pointing to intermittent MRI-driven accretion in MADs. Pinning down the nature of flux eruptions using next-generation telescopes will be crucial in understanding the flow of mass, magnetic flux and angular momentum in sub-Eddington accreting BHs like M87$^*$ and Sagittarius A$^*$.