We determine the basic physical characteristics of eight Gamma-Ray Bursts ( GRB ) – 980519 , 990123 , 990510 , 991028 , 991216 , 000301c , 000926 and 010222 – by modelling the broadband emission of their afterglows .
We find that the burst kinetic energies after the GRB phase are well clustered around a mean value of 3 \times 10 ^ { 50 } ergs .
In contrast , the energy release in \gamma -rays , after correcting for collimated explosion , varies among bursts by more than an order of magnitude .
The jet initial apertures are the 2 ^ { o } -14 ^ { o } range , mildly correlated with the energy , half of the jets being narrower than \sim 3 ^ { o } .
This implies that , within 100 Mpc , there are about 10 GRB remnants ( expanding at \sim 0.1 c ) which can be resolved with VLBA .
For all eight afterglows the total energy in the shock-accelerated electrons is close to equipartition with protons .
However the slope of the power-law electron distribution is not universal , varying between 1.4 and 2.8 .
In at least half of the cases , the density structure of the medium is inconsistent with an r ^ { -2 } profile .
A homogeneous medium with density in the 0.1 - 50 { cm ^ { -3 } } range can accommodate the broadband emission of all afterglows , with the exception of 990123 , for which we find the density to be less than 10 ^ { -2 } { cm ^ { -3 } } .
If GRBs arise from the core collapse of massive stars , then such low densities indicate the existence of superbubbles created by the supernovae and winds within a cluster of massive stars .