  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 <= Gamma <= 10^3, with <Gamma> = 370.
We find a very weak dependence of Gamma on the acceleration radius r_0, Gamma ~
r_0^alpha with alpha = -0.10 +- 0.09 at sigma = 2.1 confidence level. The
values of r_0 span a wide range, 10^6.5 <= r_0 <= 10^{9.5} cm, with mean value
<r_0>~10^{8} cm. This is higher than the gravitational radius of a 10 M_sun
black hole by a factor ~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.
