We study the properties of a significant thermal emission component that was identified in 47 GRBs observed by different instruments .
Within the framework of the “ fireball ” model , we deduce the values of the Lorentz factor \Gamma , and the acceleration radius , r _ { 0 } , for these bursts .
We find that all the values of \Gamma in our sample are in the range 10 ^ { 2 } \leq \Gamma \leq 10 ^ { 3 } , with \langle \Gamma \rangle = 370 .
We find a very weak dependence of \Gamma on the acceleration radius r _ { 0 } , \Gamma \propto r _ { 0 } ^ { \alpha } with \alpha = -0.10 \pm 0.09 at the \sigma = 2.1 confidence level .
The values of r _ { 0 } span a wide range , 10 ^ { 6.5 } \leq r _ { 0 } \leq 10 ^ { 9.5 } cm , with a mean value of \langle r _ { 0 } \rangle \sim 10 ^ { 8 } cm .
This is higher than the gravitational radius of a 10 M _ { \odot } black hole by a factor \approx 30 .
We argue that this result provides indirect evidence for jet propagation inside a massive star , and suggests the existence of recollimation shocks that take place close to this radius .