The nature of the outflow in gamma-ray bursts

P. Kumar, E. McMahon, A. Panaitescu, R. Willingale, P. O'Brien, D. Burrows, J. Cummings, N. Gehrels, S. Holland, S. B. Pandey, D. Vanden Berk, S. Zane

Research output: Contribution to journalLetterpeer-review

50 Scopus citations


The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt γ-ray emission to the afterglow phase. The early-time X-ray and optical data for GRBs obtained by telescopes aboard the Swift satellite show that the source for prompt γ-ray emission, the emission that heralds these bursts, is short lived, and is distinct from the source for the long-lived afterglow emission that follows the initial burst. Using these data we determine the distance of the γ-ray source from the centre of the explosion. We find this distance to be 1015-1016 cm for most bursts, and show that this is within a factor of about 10 of the radius of the shock heated circumstellar medium (CSM) producing the X-ray photons. Furthermore, using the early γ-ray, X-ray and optical data we show that the prompt gamma-ray emission cannot be produced in internal shocks nor can it be produced in the external shock; in a more general sense γ-ray generation mechanisms based on shock physics have problems explaining the GRB data for ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has a number of attractive features, although evidence in its favour is inconclusive. Finally, the X-ray and optical data allow us to provide an upper limit on the density of the CSM of about 10 protons cmr3 at a distance of ~5 × 1016 cm from the centre of explosion.

Original languageEnglish (US)
Pages (from-to)L57-L61
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
StatePublished - Mar 2007

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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