Distribution of gamma-ray burst ejecta energy with Lorentz factor

Jonathan Granot, Pawan Kumar

Research output: Contribution to journalLetterpeer-review


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 (Fν αt with 0.2 ≲ a ≲ 0.8) for 102.5 ≲ 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.

Original languageEnglish
Pages (from-to)L13-L16
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
StatePublished - Feb 2006
Externally publishedYes


  • Gamma-rays: bursts
  • Hydrodynamics
  • Shock waves


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