Near-horizon collisions around near-extremal black holes

Abstract Black holes have sometimes been described as astrophysical particle accelerators because finite-energy particles can collide near the horizon with divergent center-of-mass (CM) energy. The collisions are classified by the radial motion of the constituent particles at the moment of collision, with each class exhibiting a distinct near-horizon behavior. Divergence in the CM energy is sourced by the difference in the rate at which the collision radius approaches the horizon and the rate at which a particle’s angular momentum is tuned to a critical value set by the superradiant bound. To produce a high-energy collision around a near-extremal black hole (BH), at least one particle must approach criticality slower than the collision radius approaches the horizon. When both particles are ingoing or outgoing, it is additionally required that the particles approach criticality at different rates. Using a novel multi-scaling limit, we calculate the explicit form for the divergent CM energy. The angular momentum of some circular orbiters—including those at the innermost stable circular orbit radius and innermost bound circular orbit radius—approach criticality as the BH approaches extremality, indicating near-extremal BHs may be a natural environment for these high-energy collisions.