  This paper derives master equations for an atomic two-level system for a
large set of unitarily equivalent Hamiltonians without employing the rotating
wave and certain Markovian approximations. Each Hamiltonian refers to
physically different components as representing the "atom" and as representing
the "field" and hence results in a different master equation, when assuming a
photon-absorbing environment. It is shown that the master equations associated
with the minimal coupling and the multipolar Hamiltonians predict enormous
stationary state narrowband photon emission rates, even in the absence of
external driving, for current experiments with single quantum dots and colour
centers in diamond. These seem to confirm that the rotating wave Hamiltonian
identifies the components of the atom-field system most accurately.
