2006MNRAS.366L..13G

The early X-ray afterglow for a significant number of gamma-ray bursts detected by the Swift satellite is observed to have a phase of very slow flux decline with time (F_ν∝t^–α with 0.2 ≲α≲ 0.8) for 10^2.5≲t≲ 10⁴ s, while the subsequent decline is the usual 1 ≲α₃≲ 1.5 behaviour, which was seen in the pre-Swift era. We show that this behaviour is a natural consequence of a small spread in the Lorentz factor of the ejecta, by a factor of ∼2-4, where the slower ejecta gradually catch up with the shocked external medium, thus increasing the energy of the forward shock and delaying its deceleration. The end of the `shallow' flux decay stage marks the beginning of the Blandford-McKee self-similar external shock evolution. This suggests that most of the energy in the relativistic outflow is in material with a Lorentz factor of ∼30-50.