SIMBAD references

We study the properties of a significant thermal emission component that was identified in 47 gamma-ray bursts observed by different instruments. Within the framework of the ''fireball'' model, we deduce the values of the Lorentz factor Γ, and the acceleration radius, r₀, for these bursts. We find that all the values of Γ in our sample are in the range with. We find a very weak dependence of Γ on the acceleration radius r₀, with α = -0.10±0.09 at the σ = 2.1 confidence level. The values of r₀ span a wide range, cm, with a mean value of cm. This is higher than the gravitational radius of a 10 M_☉black hole by a factor of ≈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.