  Radiative transfer in a relativistic plane-parallel flow, e.g., an accretion
disk wind, is examined in the fully special relativistic treatment. Under the
assumption of a constant flow speed, for the relativistically moving atmosphere
we analytically obtain generalized Milne-Eddington solutions of radiative
moment equations; the radiation energy density, the radiative flux, and the
radiation pressure. In the static limit these solutions reduce to the
traditional Milne-Eddington ones for the plane-parallel static atmosphere,
whereas the source function nearly becomes constant as the flow speed
increases. Using the analytical solutions, we analytically integrate the
relativistic transfer equation to obtain the specific intensity. This specific
intensity also reduces to the Milne-Eddinton case in the static limit, while
the emergent intensity is strongly enhanced toward the flow direction due to
the Doppler and aberration effects as the flow speed increases (relativistic
peaking).
