Evidence on the Origin of Ergospheric Disk Field Line Topology in Simulations of Black Hole Accretion

TL;DR

This paper explores the origin of asymmetric magnetic field lines in black hole ergospheres through 3-D MHD simulations, highlighting the role of reconnection and its impact on jet power and magnetosphere flux distribution.

Contribution

It demonstrates that reconnection in the accretion disk's corona explains the asymmetric ergospheric field lines and discusses how small changes in reconnection affect jet power.

Findings

Reconnection in the accretion disk's corona influences magnetic field topology.

Small variations in reconnection geometry significantly alter jet power.

Current simulations have uncertainties due to numerical diffusion effects.

Abstract

This Letter investigates the origin of the asymmetric magnetic field line geometry in the ergospheric disk (and the corresponding asymmetric powerful jet) in 3-D perfect magnetohydrodynamic (MHD) numerical simulations of a rapidly rotating black hole accretion system reported in \citet{pun10}. Understanding, why and how these unexpected asymmetric structures form is of practical interest because an ergospheric disk jet can boost the black hole driven jet power many-fold possibly resolving a fundamental disconnect between the energy flux estimates of powerful quasar jets and simulated jet power \citep{pun11}. The new 3-D simulations of \citet{bec09} that were run with basically the same code that was used in the simulation discussed in \citet{pun10} describe the "coronal mechanism" of accreting poliodal magnetic flux towards the event horizon. It was determined that reconnection in the inner accretion disk is a "necessary" component for this process. The coronal mechanism seems to naturally explain the asymmetric ergospheric disk field lines that were seen in the simulations. Using examples from the literature, it is discussed how apparently small changes in the reconnection geometry and rates can make enormous changes in the magnetospheric flux distribution and the resultant black hole driven jet power in a numerical simulation. Unfortunately, reconnection is a consequence of numerical diffusion and not a detailed (yet to be fully understood) physical mechanism in the existing suite of perfect MHD based numerical simulations. The implication is that there is presently great uncertainty in the flux distribution of astrophysical black hole magnetospheres and the resultant jet power.