  The emission from a gamma-ray burst (GRB) photosphere can give rise to a
variety of spectral shapes. The spectrum can retain the shape of a Planck
function or it can be broadened and have the shape of a Band function. This
fact is best illustrated by studying GRB090902B: The main gamma-ray spectral
component is initially close to a Planck function, which can only be explained
by emission from the jet photosphere. Later, the same component evolves into a
broader Band function. This burst thus provides observational evidence that the
photosphere can give rise to a non-thermal spectrum. We show that such a
broadening is most naturally explained by subphotospheric dissipation in the
jet. The broadening mainly depends on the strength and location of the
dissipation, on the magnetic field strength, and on the relation between the
energy densities of thermal photons and of the electrons. We suggest that the
evolution in spectral shape observed in GRB090902B is due to a decrease of the
bulk Lorentz factor of the flow, leading to the main dissipation becoming
subphotospheric. Such a change in the flow parameters can also explain the
correlation observed between the peak energy of the spectrum and low-energy
power law slope, alpha, a correlation commonly observed in GRBs. We conclude
that photospheric emission could indeed be a ubiquitous feature during the
prompt phase in GRBs and play a decisive role in creating the diverse spectral
shapes and spectral evolutions that are observed.
