Are GRMHD Mean-Field Dynamo Models of Thick Accretion Disks SANE?

TL;DR

This paper explores whether mean-field dynamo models in axisymmetric GRMHD simulations can produce SANE-like accretion disk properties, even from unfavorable initial magnetic conditions, to better understand plasma flows around black holes.

Contribution

It demonstrates that mean-field dynamo processes can generate SANE-like accretion properties from minimal initial magnetization in axisymmetric GRMHD simulations.

Findings

Mean-field dynamo can produce SANE-like accretion features from toroidal magnetic fields.

Simulations starting with negligible magnetization can reach typical SANE disk properties.

Dynamo processes may explain observed plasma behaviors around black holes.

Abstract

The remarkable results by the Event Horizon Telescope collaboration concerning the emission from M87* and, more recently, its polarization properties, require an increasingly accurate modeling of the plasma flows around the accreting black hole. Radiatively inefficient sources such as M87* and Sgr A* are typically modeled with the SANE (standard and normal evolution) paradigm, if the accretion dynamics is smooth, or with the MAD (magnetically arrested disk) paradigm, if the black hole's magnetosphere reacts by halting the accretion sporadically, resulting in a highly dynamical process. While the recent polarization studies seem to favor MAD models, this may not be true for all sources, and SANE accretion surely still deserves attention. In this work, we investigate the possibility of reaching the typical degree of magnetization and other accretion properties expected for SANE disks by resorting to the mean-field dynamo process in axisymmetric GRMHD simulations, which are supposed to mimic the amplifying action of an unresolved magnetorotational instability-driven turbulence. We show that it is possible to reproduce the main diagnostics present in the literature by starting from very unfavorable initial configurations, such as a purely toroidal magnetic field with negligible magnetization.