The spin dependence of the Blandford-Znajek effect

TL;DR

This paper investigates how black hole spin influences the magnetic flux trapping and jet production efficiency via the Blandford-Znajek mechanism, using relativistic Kerr spacetime models to explain observations of AGN and radio galaxies.

Contribution

It introduces a flux-trapping model showing spin-dependent magnetic field enhancement on black holes, explaining observed jet efficiencies and suggesting different spin orientations power various AGN.

Findings

Lower prograde spins facilitate magnetic flux trapping.

The model explains the correlation between accretion rate and jet power.

Retrograde black holes may power the most energetic jets.

Abstract

The interaction of large scale magnetic fields with the event horizon of rotating black holes (the Blandford-Znajek [1977] mechanism) forms the basis for some models of the most relativistic jets. We explore a scenario in which the central inward "plunging" region of the accretion flow enhances the trapping of large scale poloidal field on the black hole. The study is carried out using a fully relativistic treatment in Kerr spacetime, with the focus being to determine the spin dependence of the Blandford-Znajek effect. We find that large scale magnetic fields are enhanced on the black hole compared to the inner accretion flow and that the ease with which this occurs for lower prograde black hole spin, produces a spin dependence in the Blandford-Znajek effect that has attractive applications to recent observations. Among these is the correlation between inferred accretion rate and nuclear jet power observed by Allen et al. (2006) in X-ray luminous elliptical galaxies. If the black hole rotation in these elliptical galaxies is in the prograde sense compared with that of the inner accretion disk, we show that both the absolute value and the uniformity of the implied jet-production efficiency can be explained by the flux-trapping model. The basic scenario that emerges from this study is that a range of intermediate values of black hole spins could be powering these AGN. We also suggest that the jets in the most energetic radio-galaxies may be powered by accretion onto {\it retrograde} rapidly-rotating black holes.