Open AccessThe nature of the outflow in gamma‐ray bursts
Author(s) -
Kumar P.,
McMahon E.,
Panaitescu A.,
Willingale R.,
O'Brien P.,
Burrows D.,
Cummings J.,
Gehrels N.,
Holland S.,
Pandey S. B.,
Vanden Berk D.,
Zane S.
Publication year - 2007
Publication title -
monthly notices of the royal astronomical society: letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.067
H-Index - 122
eISSN - 1745-3933
pISSN - 1745-3925
DOI - 10.1111/j.1365-2966.2007.00286.x
Subject(s) - afterglow , gamma ray burst , physics , astrophysics , radius , outflow , swift , shock (circulatory) , gamma ray , photon , vela , astronomy , optics , pulsar , medicine , computer security , meteorology , computer science
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 10 15 –10 16 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 cm −3 at a distance of ∼5 × 10 16 cm from the centre of explosion.