The nature of the outflow in gamma-ray bursts

Details The nature of the outflow in gamma-ray bursts The nature of the outflow in gamma-ray bursts

2007-02-12

arxiv.org/abs/astro-ph/0702319v1

Mon.Not.Roy.Astron.Soc.Lett.376:L57-L61,2007

Astronomy and Astrophysics

Astrophysics

Accepted to MNRAS Letters. 6 pages, 2 figures, & 2 tables

Scientific paper

10.1111/j.1365-2966.2007.00286.x

The Swift satellite has enabled us to follow the evolution of gamma-ray burst (GRB) fireballs from the prompt gamma-ray emission to the afterglow phase. The early x-ray and optical data obtained by telescopes aboard the Swift satellite show that the source for prompt gamma-ray emission, the emission that heralds these bursts, is short lived and that its source is distinct from that of the ensuing, long-lived afterglow. Using these data, we determine the distance of the gamma-ray source from the center of the explosion. We find this distance to be 1e15-1e16 cm for most bursts and we show that this is within a factor of ten of the radius of the shock-heated circumstellar medium (CSM) producing the x-ray photons. Furthermore, using the early gamma-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 gamma-ray generation mechanisms based on shock physics have problems explaining the GRB data for the ten Swift bursts analyzed in this work. A magnetic field dominated outflow model for GRBs has some attractive features, although the evidence in its favor 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 per cubic cm at a distance of about 5e16 cm from the center of explosion.

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