We investigate the quantum field aspects in flat spacetime for a uniformly accelerated observer moving in a thermal bath. In particular, we obtain an exact closed expression of the reduced density matrix for a uniformly accelerated observer with acceleration $a=2\pi T$ when the state of the quantum field is a thermal bath at temperature $T^{\prime }$. We find that the density matrix has a simple form with an effective partition function $Z$ being a product, $Z=Z_T{Z}_{T^{\prime }}$, of two thermal partition functions corresponding to temperatures $T$ and $T^{\prime }$ and hence is not thermal, even when $T=T^{\prime }$. We show that, even though the partition function has a product structure, the two thermal baths are, in fact, interacting systems; although in the high frequency limit ${\omega }_k\gg T$ and ${\omega }_k\gg T^{\prime }$, the interactions are found to become subdominant. We further demonstrate that the resulting spectrum of the Rindler particles can be interpreted in terms of spontaneous and stimulated emission due to the background thermal bath. The density matrix is also found to be symmetric in the acceleration temperature $T$ and the thermal bath temperature $T^{\prime }$ indicating that thermodynamic experiments alone cannot distinguish between the thermal effects due to $T$ and those due to $T^{\prime }$. The entanglement entropy associated with the reduced density matrix (with the background contribution of the Davies-Unruh bath removed) is shown to satisfy, in the ${\omega }_k\gg T^{\prime }$ limit, a first law of thermodynamics relation of the form $T\delta S=\delta E$, where $\delta E$ is the difference in the energies corresponding to the reduced density matrix and the background Davies-Unruh bath. The implications are discussed.

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