  We consider the emission of photons from the inner parts of a
relativistically expanding plasma outflow, characterized by a constant Lorentz
factor, Gamma. Photons that are injected in regions of high optical depth are
advected with the flow until they escape at the photosphere. Due to multiple
scattering below the photosphere, the locally emerging comoving photon
distribution is thermal. However, as an observer sees simultaneously photons
emitted from different angles, hence with different Doppler boosting, the
observed spectrum is a multi-color black-body. We calculate here the properties
of the observed spectrum at different observed times. Due to the strong
dependence of the photospheric radius on the angle to the line of sight, for
parameters characterizing gamma-ray bursts (GRBs) thermal photons are seen up
to tens of seconds following the termination of the inner engine. At late
times, following the inner engine termination, both the number flux and energy
flux of the thermal spectrum decay as F ~ t^{-2}. At these times, the
multicolor black body emission results in a power law at low energies (below
the thermal peak), with power law index F_\nu ~ \nu^{0}. This result is
remarkably similar to the average value of the low energy spectral slope index
(``\alpha'') seen in fitting the spectra of large GRB sample.

High Energy Astrophysical Phenomena