Black hole magnetosphere with small scale flux tubes

TL;DR

This paper explores how small-scale magnetic flux tubes near a black hole can influence the structure of the magnetosphere and potentially power the compact X-ray emitting corona observed in some Seyfert galaxies.

Contribution

It models the black hole magnetosphere with small-scale flux tubes using the force-free approximation, revealing how the closed zone size depends on black hole spin and external pressure.

Findings

The extent of the closed flux tube zone is typically a few gravitational radii.

Balance between black hole spin-induced twist and external pressure determines the closed zone size.

Pressure dynamics may lead to dissipation mechanisms relevant for X-ray corona formation.

Abstract

There is observational evidence that the X-ray continuum source that creates the broad fluorescent emission lines in some Seyfert Galaxies may be compact and located at a few gravitational radii above the black hole. We consider the possibility that this compact source may be powered by small scale flux tubes near the black hole that are attached to the orbiting accretion disk. As a first step, this paper investigates the salient features of black hole magnetospheres that contain small scale, disk-hole linking "closed" flux tubes, using the force-free approximation in an axisymmetric setting. We find that the extent of the closed zone is a result of the balance between the black hole spin induced twist in the closed zone and the confinement pressure of the external (open) field of the disk. The maximal extent of the closed zone, for a typical external confinement, is usually a few gravitational radii. The pressure competition between the closed zone and the external confinement could in principle lead to interesting dynamics and dissipation relevant for the compact X-ray corona.