Magnetic field lines entering the event horizon of a rapidly spinning black hole can slow down its rotation, effectively acting like rubber bands that are dragged along with it. This process is a well known theoretical concept under the name of the Blandford/Znajek process. Though this theoretical model appeals through its clarity in simple equilibrium solutions of magnetic fields around black holes, it was unclear if and how it operates in vastly dynamical scenarios. Such a scenario can be the accretion of a tube of magnetic flux onto a black hole, as an alternative to the continuous build-up of large-scale magnetic structures. In this research, we extend readily available results from 2D simulations to prove that episodes of an efficiently operating Blandford/Znajek process emerge in carefully modeled 3D magnetospheres. We use the variability constraints obtained by finding optimal sizes for the magnetic flux tubes to relate the variability of efficient energy extraction from the central black hole to actual astronomical observations. For a black hole like the one in the center of the galaxy M87 we estimate variability time scales of several days to weeks for polarity reversals during the accretion of magnetic loops.
Research summary
- We devised a way to feed magnetic loops onto a rapidly spinning black hole by adding current driven layers and resistive transition regions around an in-situ accretion disk model in a force-free Kerr magnetosphere.
- We then conducted a parameter study to examine the dynamics of magnetic loops that fall onto the central object with different extensions and accretion velocities. Especially for counter-rotating accretion disks with an innermost circular orbit at some distance from the event horizon we were able to deduce an ideal loop geometry for the launch of efficient Blandford/Znajek episodes.
- Our main argumentation in this review of efficient energy extraction relies on the comparison of time scales given by the disk rotation vs. the rotation of the black hole itself. While field lines slip across the event horizon, they need sufficient time to wind up and successfully produce a jet-like structure. With insufficient time, failed jet structures can tip over on the disk and do not fuel a powerful striped jet with each accreted loop in three dimensions.
- We also showed that even in 3D and an extremely dynamic environment, where many assumptions of the original derivation of the Blandford/Znajek process do not hold, ideal conditions are reached at the black hole event horizon whenever efficient outflows occur. We conclude that it can operate quite universally in arbitrary geometries whenever there are outgoing Poynting (electromagnetic energy) fluxes from a rapidly spinning black hole immersed in a force-free magnetosphere.
Visualizing science
During the accretion of a magnetic flux tube, the magnetosphere around a rapidly spinning Kerr black hole becomes highly ordered and jet-like structures of outgoing Poynting flux (ribbons) emerge. Once the polarity of the in-falling magnetic fields (color density) changes, the extended structure is lost. An inefficient phase of magnetospheric re-polarization takes over from an efficient Blandford/Znajek episode. This process repeats itself and produces pulses of efficient energy extraction from the central black hole. The pulses have a duration of several days to weeks if scaled to a black hole like the one at the center of the M87 galaxy and when assuming the ideal loop geometry we postulated in this work.
Collaborative results
J. F. Mahlmann, A. Levinson, M. A. Aloy, Striped Blandford/Znajek jets from advection of small-scale magnetic field, Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 4203–4225, https://doi.org/10.1093/mnras/staa943.
Jens Mahlmann 