Magnetized advective accretion flows: formation of magnetic barriers in Magnetically Arrested Discs

TL;DR

This paper explores how strong magnetic fields in accretion disks around black holes can create magnetic barriers that influence matter flow, jet formation, and episodic outflows, using a simplified 1.5D model.

Contribution

It introduces the concept of magnetic barriers in magnetically arrested disks and analyzes their role in matter dynamics and jet formation near black holes.

Findings

Magnetic barriers can decelerate or halt accreting matter near the event horizon.

Matter can be trapped between multiple magnetic barriers, leading to episodic jets.

Magnetic fields from the environment can be advected inward, influencing disk behavior.

Abstract

We discuss the importance of large scale strong magnetic field in the removal of angular momentum outward, as well as the possible origin of different kinds of magnetic barrier in advective, geometrically thick, sub-Keplerian accretion flows around black holes. The origin of this large scale strong magnetic field near the event horizon is due to the advection of the magnetic flux by the accreting gas from the environment, say, the interstellar medium or a companion star, because of flux freezing. In this simplest vertically averaged, 1.5-dimensional disc model, we choose the maximum upper limit of the magnetic field, which the disc around a black hole can sustain. In this so called magnetically arrested disc (MAD) model, the accreting gas either decelerates or faces the magnetic barrier near the event horizon by the accumulated magnetic field depending on the geometry. The magnetic barrier may knock the matter to infinity. We suggest that these types of flow are the building block to produce jets and outflows in the accreting system. We also find that in some cases, when matter is trying to go back to infinity after knocking the barrier, matter is prevented being escaped by the cumulative action of strong gravity and the magnetic tension, hence by another barrier. In this way, magnetic field can lock the matter in between these two barriers and it might be a possible explanation for the formation of episodic jet.