Kinematics of ultra-high energy particle collisions near black holes in the magnetic field

There are different versions of collisions of two particles near black holes with unbound energy $E_{c.m.}$ in the centre of mass frame. The so-called BSW effect arises when a slow fine-tuned "critical" particle hits a rapid "usual" one. We discuss a scenario of collision in the strong magnetic field for which explanation tuns out to be different. Both particles are rapid but the nonzero angle between their velocities (which are both close to $c$, the speed of light) results in a relative velocity close to $c$ and, hence, big $% E_{c.m.}