Magnetic black holes: From Thomson dipoles to the Penrose process and cosmic censorship

We consider accretion of charged test matter by rotating, magnetic black holes and discuss a number of aspects in which the interaction of the angular momentum contained in the electromagnetic field and the spin of the hole plays a fundamental role. First, we argue that such a black hole tends to lose its angular momentum by accreting charges while remaining globally neutral. Then, we show that accretion can happen in a superradiant manner via an enhanced Penrose process. In particular, we find that the regions from which energy and angular momentum can be extracted contain the axis of rotation and, in some cases, consist of floating bubbles disconnected from the black hole itself. Finally, we address the question of whether extremal dyonic rotating black holes can be overcharged or overspun via accretion of arbitrary matter and prove that this cannot happen if the null-energy condition holds. We conclude by discussing some future research directions.