Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway towards a Magnetically Arrested Disk

TL;DR

This study uses 3D GRMHD simulations to explore how large-scale magnetic fields are transported inward in radiatively inefficient accretion flows, highlighting the potential for forming magnetically arrested disks and strong jets.

Contribution

It demonstrates that magnetic flux can be efficiently transported inward in RIAFs via external field loops, suggesting a pathway for MAD formation.

Findings

A significant fraction (15-40%) of injected flux reaches the black hole.

Flux transport efficiency is high and weakly dependent on loop parameters.

External magnetic fields can facilitate MAD formation in RIAFs.

Abstract

Large-scale magnetic fields play a vital role in determining the angular momentum transport and generating jets/outflows in the accreting systems, yet their origin remains poorly understood. We focus on radiatively inefficient accretion flows (RIAFs) around the black holes (BHs), and conduct three-dimensional general-relativistic magnetohydrodynamic (GRMHD) simulations using the Athena++ code. We first re-confirm that the MRI dynamo in the RIAF alone does not spontaneously form a magnetically arrested disk (MAD), conducive for strong jet formation. We next investigate the other possibility, where the large-scale magnetic fields are advected inward from external sources (e.g. the companion star in X-ray binaries, magnetized ambient medium in AGNs). Although the actual configuration of the external fields could be complex and uncertain, they are likely to be closed. As a first study, we treat them as closed field loops of different sizes, shapes and field strengths. Unlike earlier studies of flux transport, where magnetic flux is injected in the initial laminar flow, we injected the magnetic field loops in the quasi-stationary turbulent RIAF in inflow equilibrium and followed their evolution. We found that a substantial fraction ($\sim15\%-40\%$) of the flux injected at the large radii reaches the BH with a weak dependence on the loop parameters except when the loops are injected at high latitudes, away from the mid-plane. Relatively high efficiency of flux transport observed in our study hints that a MAD might be formed relatively easily close to the BH, provided that a source of the large-scale field exists at the larger radii.