The Cosmic Battery and the Inner Edge of the Accretion Disk

TL;DR

This paper investigates the stability and structural changes of the inner accretion disk around black holes due to magnetic fields generated by the Poynting-Robertson Cosmic Battery, highlighting how magnetic field growth affects disk dynamics.

Contribution

It provides a theoretical analysis of the magnetic field's impact on the inner edge of accretion disks, incorporating general relativity and the Cosmic Battery mechanism, which is a novel approach.

Findings

Inner disk edge moves outward as magnetic field grows.

In fast-spinning black holes, the inner edge can move inward near the horizon.

The inner disk undergoes significant structural changes near magnetic equipartition.

Abstract

The Poynting-Robertson Cosmic Battery proposes that the innermost part of the accretion disk around a black hole is threaded by a large scale dipolar magnetic field generated in situ, and that the return part of the field diffuses outward through the accretion disk. This is different from the scenario that the field originates at large distances and is carried inward by the accretion flow. In view of the importance of large scale magnetic fields in regulating the processes of accretion and outflows, we study the stability of the inner edge of a magnetized disk in general relativity when the distribution of the magnetic field is the one predicted by the Poynting-Robertson Cosmic Battery. We found that as the field grows, the inner edge of the disk gradually moves outward. In a fast spinning black hole with a>0.8M the inner edge moves back in towards the black hole horizon as the field grows beyond some threshold value. In all cases, the inner part of the disk undergoes a dramatic structural change as the field approaches equipartition.