  We formulate and solve the diffusion problem of line photon propagation in a
bulk outflow from a compact object (black hole or neutron star) using a generic
assumption regarding the distribution of line photons within the outflow.
Thomson scattering of the line photons within the expanding flow leads to a
decrease of their energy which is of first order in v/c, where v is the outflow
velocity and c is the speed of light. We demonstrate that the emergent line
profile is closely related to the time distribution of photons diffusing
through the flow (the light curve) and consists of a broad redshifted feature.
We analyzed the line profiles for the general case of outflow density
distribution. We emphasize that the redshifted lines are intrinsic properties
of the powerful outflow that are supposed to be in many compact objects.
