On white holes as particle accelerators

We analyze scenarios of particle collisions in the metric of a nonextremal black hole that can potentially lead to ultrahigh energy $E_{c.m.}$ in their centre of mass frame. Particle 1 comes from infinity to the black hole horizon while particle 2 emerges from a white hole region. It is shown that unbounded $E_{c.m.}$ $\$require that particle 2 pass close to the bifurcation point. The analogy with collisions inside the horizon is discussed.