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