Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2009-08-05
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
6 pages, 7 figures, to be published in the proceedings of the conference: Neutron Stars and Gamma Ray Bursts: Recent Developme
Scientific paper
We call "prompt" emission of Gamma Ray Bursts (GRBs) the erratic and violent phase of hard X-ray and soft gamma-ray emission, usually lasting for tens of seconds in long GRBs. However, the central engine of GRBs may live much longer. Evidence of it comes from the strange behaviour of the early "afterglow", seen especially in the X-ray band, characterised by a "steep-flat-steep" light curve, very often not paralleled by a similar behaviour in the optical band. This difference makes it hard to explain both the optical and the X-ray emission with a unique component. Two different mechanisms seem to be required. One can well be the standard emission from the forward shock of the fireball running through the interstellar medium. The second one is more elusive, and to characterise its properties we have studied those GRBs with well sampled data and known redshift, fitting at the same time the optical and the X-ray light curves by means of a composite model, i.e. the sum of the standard forward shock emission and a phenomenological additional component with a minimum of free parameters. Some interesting findings have emerged, pointing to a long activity of the central engine powered by fallback material.
Ghirlanda Giancarlo
Ghisellini Gabriele
Nardini Marco
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