  We consider radiative processes of a quantum system composed by two identical
two-level atoms in a black-hole background. We assume that these identical
two-level atoms are placed at fixed radial distances outside a Schwarzschild
black hole and interacting with a quantum electromagnetic field prepared in one
of the usual vacuum states, namely the Boulware, Unruh or the Hartle-Hawking
vacuum states. We study the structure of the rate of variation of the atomic
energy. The intention is to identify in a quantitative way the contributions of
vacuum fluctuations and radiation reaction to the entanglement generation
between the atoms as well as the degradation of entangled states in the
presence of an event horizon. We find that for a finite observation time the
atoms can become entangled for the case of the field in the Boulware vacuum
state, even if they are initially prepared in a separable state. In addition,
the rate of variation of atomic energy is not well behaved at the event horizon
due to the behavior of the proper accelerations of the atoms. We show that the
thermal nature of the Hartle-Hawking and Unruh vacuum state allows the atoms to
get entangled even if they were initially prepared in the separable ground
state.
