In this model a collimated ultra-relativistic ejecta collides with an amorphous dense cloud surrounding the central engine , producing gamma-rays via synchrotron process .
The ejecta is taken as a standard candle , while assuming a gaussian distribution in thickness and density of the surrounding cloud .
Due to the cloud high density , the synchrotron emission would be an instantaneous phenomenon ( fast cooling synchrotron radiation ) , so a GRB duration corresponds to the time that the ejecta takes to pass through the cloud .
Fitting the model with the observed bimodal distribution of GRBs ’ durations , the ejecta ’ s initial Lorentz factor , and its initial opening angle are obtained as \Gamma _ { 0 } \lesssim 10 ^ { 3 } , and \zeta _ { 0 } \approx 10 ^ { -2 } , and the mean density and mean thickness of the surrounding cloud as \overline { n } \sim 3 \times 10 ^ { 17 } cm ^ { -3 } and \overline { L } \sim 2 \times 10 ^ { 13 } cm .
The clouds maybe interpreted as the extremely amorphous envelops of Thorne-Zytkow objects .
In this model the two classes of long and short duration GRBs are explained in a unique frame .