  We treat an Unruh-DeWitt detector as an open quantum system and evaluate the
response of a uniformly accelerated detector: (i) interacting locally with the
derivatives of a massless scalar field and (ii) linearly coupled to an
electromagnetic field. We find that the early-time transition rate of the
detector strongly depends on the type of the interaction between the detector
and the quantum field, and may not follow a Planck distribution. In contrast,
the late time asymptotic state is always thermal at the Unruh temperature and
thus provides a more fundamental and persistent characterization of the
acceleration temperature: A uniformly accelerated detector experiences the
field vacuum as a genuine thermal bath at the Unruh temperature and eventually
settles at a thermal state, regardless of their intermediate dynamics or the
type of interaction.
