Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-01-16
Astronomy and Astrophysics
Astrophysics
Accepted for publication in ApJ Letters
Scientific paper
10.1086/515570
The flat decay phase in the first 1e2-1e4 seconds of the X-ray light curve of Gamma Ray Bursts (GRBs) has not yet found a convincing explanation. The fact that the optical and X-ray lightcurves are often different, with breaks at different times, makes contrived any explanation based on the same origin for both the X-ray and optical fluxes. We here assume that the central engine can be active for a long time, producing shells of decreasing bulk Lorentz factors Gamma. We also assume that the internal dissipation of these late shells produces a continuous and smooth emission (power-law in time), usually dominant in X-rays and sometimes in the optical. When Gamma of the late shells is larger than 1/theta_j, where theta_j is the jet opening angle, we see only a portion of the emitting surface. Eventually, Gamma becomes smaller than 1/theta_j, and the entire emitting surface is visible. Thus there is a break in the light curve when Gamma=1/theta_j, which we associate to the time at which the plateau ends. After the steeply decaying phase which follows the early prompt, we see the sum of two emission components: the "late-prompt" emission (due to late internal dissipation), and the "real afterglow" emission (due to external shocks). A variety of different optical and X-ray light curves are then possible, explaining why the X-ray and the optical light curves often do not track each other (but sometimes do), and often they do not have simultaneous breaks.
Firmani Claudio
Ghirlanda Giancarlo
Ghisellini Gabriele
Nava Lara
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