  Using the formalism that separates the contributions of vacuum fluctuations
and radiation reaction to the rate of change of the mean atomic energy, we show
that a two-level atom in interaction with a quantum massless scalar field both
in the Hartle-Hawking and Unruh vacuum in a 1+1 dimensional black hole
background spontaneously excites as if there is thermal radiation at the
Hawking temperature emanating from the black hole. Our calculation, therefore,
ties the existence of Hawking radiation to the spontaneous excitation of a
two-level atom placed in vacuum in the exterior of a black hole and shows
pleasing consistence of two different physical phenomena, the Hawking radiation
and the spontaneous excitation of atoms, which are quite prominent in their own
right.
