We present an analysis of the Internal Shock Model of GRBs , where gamma-rays are produced by internal shocks within a relativistic wind .
We show that observed GRB characteristics impose stringent constraints on wind and source parameters .
We find that a significant fraction , of order 20 % , of the wind kinetic energy can be converted to radiation , provided the distribution of Lorentz factors within the wind has a large variance and provided the minimum Lorentz factor is > \Gamma _ { \pm } \approx 10 ^ { 2.5 } L _ { 52 } ^ { 2 / 9 } , where L = 10 ^ { 52 } L _ { 52 } { erg s } ^ { -1 } is the wind luminosity .
For a high , > 10 \% , efficiency wind , spectral energy breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time R / c \lesssim 1 ms , suggesting a possible explanation for the observed clustering of spectral break energies in this range .
The lower limit \Gamma _ { \pm } to wind Lorenz factor and the upper limit \approx 1 ( R / 10 ^ { 7 } { cm } ) ^ { -5 / 6 } MeV to observed break energies are set by Thomson optical depth due to e ^ { \pm } pairs produced by synchrotron photons .
Natural consequences of the model are absence of bursts with peak emission energy significantly exceeding 1 MeV , and existence of low luminosity bursts with low , 1 keV to 10 keV , break energies .