  We study the Hawking radiation in the framework of open quantum systems by
examining the time evolution of a detector (modelled by a two-level atom)
interacting with vacuum massless scalar fields. The dynamics of the detector is
governed by a master equation obtained by tracing over the field degrees of
freedom from the complete system. The nonunitary effects are studied by
analyzing the time behavior of a particular observable of the detector, i.e.,
its admissible state, in the Unruh, Hartle-Hawking, as well as Boulware vacua
outside a Schwarzschild black hole. We find that the detector in both the Unruh
and Hartle-Hawking vacua would spontaneously excite with a nonvanishing
probability the same as what one would obtain if there is thermal radiation at
the Hawking temperature from the black hole, thus reproducing the basic results
concerning the Hawking effect in the framework of open quantum systems.
