Publikace ÚTF

In 2009, Bañados, Silk, and West (BSW) pointed out the possibility of having an unbounded limit of
center-of-mass collision energy for test particles in the field of an extremal Kerr black hole, if one of them
has fine-tuned parameters and the collision point is approaching the horizon. The possibility of this “BSW
effect” attracted much attention: it was generalized to arbitrary (“dirty”) rotating black holes and an analogy
was found for collisions of charged particles in the field of nonrotating charged black holes. Our work
considers the unification of these two mechanisms, which have so far been studied only separately.
Exploring the enlarged parameter space, we find kinematic restrictions that may prevent the fine-tuned
particles from reaching the limiting collision point. These restrictions are first presented in a general form,
which can be used with an arbitrary black-hole model, and then visualized for the Kerr-Newman solution
by plotting the “admissible region” in the parameter space of critical particles, reproducing some known
results and obtaining a number of new ones. For example, we find that (marginally) bounded critical
particles with enormous values of angular momentum can, curiously enough, approach the degenerate
horizon, if the charge of the black hole is very small. Such “mega-BSW” behavior is excluded in the case of
a vacuum black hole, or a black hole with large charge. It may be interesting in connection with the small
“Wald charge” induced on rotating black holes in external magnetic fields.