[neutralino]

Apparent horizons are normally mentioned when discussing the formation of black holes. See here for some brief comments. The important difference between an absolute and an apparent horizon is that just because a particle is outside the apparent horizon at some moment in time, does not mean that it will be able to escape the black hole.

Note that the terms 'event horizon' and 'absolute horizon' are usually synonymous.

I'm on vacation at the moment, so unfortunately can't answer in detail. I'll get onto it in the new year when I'm back home.