Magnetic Fields Threading Black Holes: restrictions from general relativity and implications for astrophysical black holes

TL;DR

This paper examines the limitations of current models of electromagnetic fields around black holes, highlighting discrepancies between analytic and numerical results, and suggests that accretion orientation influences jet strength.

Contribution

It identifies potential issues in GRMHD simulations and demonstrates from first principles that accretion direction affects the strength of black hole-threading magnetic fields.

Findings

Analytic and numerical results show tension in modeling black hole magnetospheres.

Prograde accretion likely results in weaker magnetic fields and jets.

Retrograde accretion may produce stronger electromagnetic interactions.

Abstract

The idea that black hole spin is instrumental in the generation of powerful jets in active galactic nuclei and X-ray binaries is arguably the most contentious claim in black hole astrophysics. Because jets are thought to originate in the context of electromagnetism, and the modeling of Maxwell fields in curved spacetime around black holes is challenging, various approximations are made in numerical simulations that fall under the guise of 'ideal magnetohydrodynamics'. But the simplifications of this framework may struggle to capture relevant details of real astrophysical environments near black holes. In this work, we highlight tension between analytic and numerical results, specifically between the analytically derived conserved Noether currents for rotating black hole spacetimes and the results of general relativistic numerical simulations (GRMHD). While we cannot definitively attribute the issue to any specific approximation used in the numerical schemes, there seem to be natural candidates, which we explore. GRMHD notwithstanding, if electromagnetic fields around rotating black holes are brought to the hole by accretion, we show from first principles that prograde accreting disks likely experience weaker large-scale black hole-threading fields, implying weaker jets than in retrograde configurations.