Dynamic Boundaries of Event Horizon Magnetospheres

TL;DR

This paper uses 3D simulations to study the magnetic flux and Poynting flux in black hole magnetospheres, revealing that boundaries significantly influence energy flux and challenge traditional assumptions about the Blandford-Znajek mechanism.

Contribution

It demonstrates that boundary effects in 3D MHD simulations can dominate Poynting flux injection, contrasting with classical models assuming boundary insensitivity.

Findings

Strong boundary-driven Poynting flux at high spins

Boundary effects can overshadow Blandford-Znajek effects

Lower spins show modest Poynting flux from accretion disk corona

Abstract

This Letter analyzes 3-dimensional simulations of Kerr black hole magnetospheres that obey the general relativistic equations of perfect magnetohydrodynamics (MHD). Particular emphasis is on the event horizon magnetosphere (EHM) which is defined as the the large scale poloidal magnetic flux that threads the event horizon of a black hole (This is distinct from the poloidal magnetic flux that threads the equatorial plane of the ergosphere, which forms the ergospheric disk magnetosphere). Standard MHD theoretical treatments of Poynting jets in the EHM are predicated on the assumption that the plasma comprising the boundaries of the EHM plays no role in producing the Poynting flux. The energy flux is electrodynamic in origin and it is essentially conserved from the horizon to infinity, this is known as the Blandford-Znajek (B-Z) mechanism. To the contrary, within the 3-D simulations, the lateral boundaries are strong pistons for MHD waves and actually inject prodigious quantities of Poynting flux into the EHM. At high black hole spin rates, strong sources of Poynting flux adjacent to the EHM from the ergospheric disk will actually diffuse to higher latitudes and swamp any putative B-Z effects. This is in contrast to lower spin rates, which are characterized by much lower output powers and modest amounts of Poynting flux are injected into the EHM from the accretion disk corona.