  A static observer with a finite lifetime has causal access to only a limited
region of spacetime known as the causal diamond. The presence of an apparent
horizon in the causal diamond, due to the observer's finite lifetime, is the
origin of an Unruh-like thermal effect. Thus, even though the observer is
static and the background is flat, the finite-lifetime observer experiences a
thermal bath in the Minkowski vacuum. In this article, we provide an open
quantum systems approach that yields a complete thermal characterization via
the observer's steady-state density matrix, which is shown to be thermal with a
temperature inversely proportional to its lifetime. This associated diamond
temperature agrees with the established result derived from other methods.
Moreover, our approach is particularly useful for designing entanglement
harvesting protocols in the causal diamond. In addition, we introduce an
insightful procedure that defines diamond coordinates using conformal
transformations, and which leads to a more direct derivation of the thermal
properties.
