On the Universality of Energy Extraction from Black Hole Spacetimes

TL;DR

This study investigates whether the Blandford-Znajek mechanism for jet power extraction from black holes remains valid in generalized gravity theories, revealing that jet power can significantly deviate from Kerr predictions due to frame-dragging effects.

Contribution

It provides the first comprehensive analysis of energy extraction via the BZ mechanism in non-Kerr black hole spacetimes using 3D MHD simulations, uncovering new degeneracies in black hole parameter inference.

Findings

BZ jet power relation holds in generalized spacetimes.

Frame-dragging variation can enhance or suppress jet power.

Degeneracy in BH parameter inference due to frame-dragging effects.

Abstract

The launching of astrophysical jets provides the most compelling observational evidence for direct extraction of black hole (BH) spin energy via the Blandford-Znajek (BZ) mechanism. Whilst it is known that spinning Kerr BHs within general relativity (GR) follow the BZ jet power relation, the nature of BH energy extraction in general theories of gravity has not been adequately addressed. This study performs the first comprehensive investigation of the BZ jet power relation by utilizing a generalized BH spacetime geometry which describes parametric deviations from the Kerr metric of GR, yet recovers the Kerr metric in the limit that all deviation parameters vanish. Through performing and analyzing an extensive suite of three-dimensional covariant magnetohydrodynamics (MHD) simulations of magnetized gas accretion onto these generalized BH spacetimes we find that the BZ jet power relation still holds, in some instances yielding jet powers far in excess of what can be produced by even extremal Kerr BHs. It is shown that independent variation of the frame-dragging rate of the BH can enhance or suppress the effects of BH spin, and by extension of frame-dragging. This variation greatly enhances or suppresses the observed jet power and underlying photon ring image asymmetry, introducing a previously unexplored yet important degeneracy in BH parameter inference. Finally we show that sufficiently accurate measurements of the jet power, accretion rate and photon ring properties from supermassive BHs can potentially break this degeneracy, highlighting the need of independent investigations of BH frame-dragging from observations.