Evidence on the origin of ergospheric disc field line topology in simulations of black hole accretion

This Letter investigates the origin of the asymmetric magnetic field line geometry in the ergospheric disc (and the corresponding asymmetric powerful jet) in 3D perfect magnetohydrodynamic (MHD) numerical simulations of a rapidly rotating black hole accretion system reported in Punsly, Igumenshchev & Hirose. Understanding why and how these unexpected asymmetric structures form is of practical interest because an ergospheric disc jet can boost the black hole driven jet power many fold, possibly resolving a fundamental disconnect between the energy flux estimates of powerful quasar jets and simulated jet power. The new 3D simulations of Beckwith, Hawley & Krolik that were run with basically the same code that was used in the simulation discussed in Punsly et al. describe the ‘coronal mechanism’ of accreting poloidal magnetic flux towards the event horizon. It was determined that reconnection in the inner accretion disc is a ‘necessary’ component for this process. The coronal mechanism seems to naturally explain the asymmetric ergospheric disc field lines that were seen in the simulations. Using examples from the literature, it is discussed how apparently small changes in the reconnection geometry and rates can make enormous changes in the magnetospheric flux distribution and the resultant black hole driven jet power in a numerical simulation. Unfortunately, reconnection is a consequence of numerical diffusion and not a detailed (yet to be fully understood) physical mechanism in the existing suite of perfect MHD-based numerical simulations. The implication is that there is presently great uncertainty in the flux distribution of astrophysical black hole magnetospheres and the resultant jet power.