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We present spectral calculations for Gamma Ray Bursts (GRB) capable of explaining the bulk-part of the bursts' spectral properties. Relativistic outflows are invoked by many GRB models. Here, we consider the unsteady relativistic wind (Rees and M\'esz\'aros, 1994) in a cosmological framework. We stress its ability to produce up to three bursts with distinct spectra, and examine them, in the context of explaining bursts like the exceptional burst of 2/17/1994, as well as the more typical ones.
The optically thick wind, that lasts for $t_{w}$ and varies on $t_{var}$, consists mainly of radiation with some baryonic contamination. When relativistic expansion ($\Gamma \sim 10^{2}$) turns the flow optically thin, a burst approximating a black body spectrum of $T_{eff} \sim 0.1- 10 keV$ with $m_{bol} > 9$ occurs, lasts for $t_{w}$ and goes undetected (in most cases). The bulk of the energy will be dissipated and radiated away later, partly when,due to the flow being unsteady, internal shocks develop and partly when the swept-up surrounding material decelerates the flow causing the formation of a blast wave and a reverse shock. The former burst lasts for $t_{w}$ while the latter takes the typical expansion time-scale ($t_{ex}$).
Those two bursts will occur with a time difference of $2 t_{ex}$ and will both have non-thermal spectra. The shocks accelerate electrons and carry frozen-in magnetic field and/or turbulently generate it, thus giving rise to radiation via synchrotron and inverse Compton scattering processes. The power indices and spectral break frequencies are in good agreement with observations.
The delayed burst will be, in general, brighter and more energetic (up to $10^{2} GeV$) than the one due to internal shocks ($10^{2} keV- GeV$). The latter only, or both bursts, are expected to have low energy tails (X-ray down to UV) that may be detectable. A steady wind will not produce the 'internal shock' burst, and a flow that is very poor in baryonic contaminants will result in a thermal burst only thus covering the full range of the bursts' observed spectral characteristics.
