  An exact reduced dynamical map along with its operator sum representation is
derived for a central spin interacting with a thermal spin environment. The
dynamics of the central spin shows high sustainability of quantum traits such
as coherence and entanglement in the low-temperature regime. However, for
sufficiently high temperature and when the number of bath particles approaches
the thermodynamic limit, this feature vanishes and the dynamics closely mimics
Markovian evolution. The properties of the long-time-averaged state and the
trapped information of the initial state for the central qubit are also
investigated in detail, confirming that the nonergodicity of the dynamics can
be attributed to the finite temperature and finite size of the bath. It is
shown that if a certain stringent resonance condition is satisfied, the
long-time-averaged state retains quantum coherence, which can have far reaching
technological implications in engineering quantum devices. An exact time-local
master equation of the canonical form is derived. With the help of this master
equation, the nonequilibrium properties of the central spin system are studied
by investigating the detailed balance condition and irreversible entropy
production rate. The result reveals that the central qubit thermalizes only in
the limit of very high temperature and large number of bath spins.
