  The energy and particle fluxes emitted by an accelerated two level atom are
analysed in detail. It is shown both perturbatively and non perturbatively that
the total number of emitted photons is equal to the number of transitions
characterizing thermal equilibrium thereby confirming that each internal
transition is accompanied by the emission of a Minkowski quantum. The mean
fluxes are then decomposed according to the final state of the atom and the
notion of conditional flux is introduced. This notion is generalized so as to
study the energy content of the vacuum fluctuations that induce the transitions
of the accelerated atom. The physical relevance of these conditional fluxes is
displayed and contact is made with the formalism of Aharonov et al. The same
decomposition is then applied to isolate, in the context of black hole
radiation, the energy content of the particular vacuum fluctuations which are
converted into on mass shell quanta. It is shown that initially these
fluctuations are located around the light like geodesic that shall generate the
horizon and have exponentially large energy densities. Upon exiting from the
star they break up into two pieces. The external one is red shifted and becomes
an on mass shell quantum, the other, its ''partner", ends up in the
singularity. We avail ourselves of this analysis to study back reaction effects
to the production of a single quantum.
