Prograde and Retrograde Black Holes: Whose Jet is More Powerful?

TL;DR

This study uses 3D GRMHD simulations to compare magnetic flux trapping and jet efficiency in prograde and retrograde black holes, revealing prograde BHs trap more flux and produce more powerful jets than previously predicted.

Contribution

It provides the first self-consistent simulation-based comparison of magnetic flux trapping and jet efficiency between prograde and retrograde black holes.

Findings

Prograde BHs trap more magnetic flux than retrograde BHs.

Jet efficiency can reach about 100% for both spin orientations.

The similarity in jet power complicates inferring BH spin direction from jet energetics.

Abstract

The outflow efficiency (eta) from black hole (BH) accretion disc systems is known to depend upon both the BH spin (a) and the amount of large-scale magnetic flux threading the BH and disc. Semi-analytical flux-trapping models suggest retrograde BHs should trap much more large-scale magnetic flux near the BH leading to much higher eta than for prograde BHs. We self-consistently determine the amount of large-scale magnetic flux trapped by rapidly spinning (a = -0.9 and 0.9) BHs using global 3D time-dependent non-radiative general relativistic magnetohydrodynamic simulations of thick (h/r ~ 0.3-0.6) discs. We find that BH-trapped flux builds up until it is strong enough to disrupt the inner accretion disc. Contrary to prior flux-trapping models, which do not include the back-reaction of magnetic flux on the disc, our simulations show prograde BHs trap more magnetic flux, leading to about 3 times higher eta than retrograde BHs for |a| = 0.9. Both spin orientations can produce highly efficient jets, eta ~ 100%, with increasing eta for increasing disc thickness. The similarity of eta for prograde and retrograde BHs makes it challenging to infer the sign of BH spin based on jet energetics alone.