  The central engine of Gamma Ray Bursts may live much longer than the duration
of the prompt emission. Some evidence of it comes from the presence of strong
precursors, post-cursors, and X-ray flares in a sizable fraction of bursts.
Additional evidence comes from the fact that often the X-ray and the optical
afterglow light curves do not track one another, suggesting that they are two
different emission components. The typical "steep-flat-steep" behavior of the
X-ray light curve can be explained if the same central engine responsible for
the main prompt emission continues to be active for a long time, but with a
decreasing power. The early X-ray "afterglow" emission is then the extension of
the prompt emission, originating at approximately the same location, and is not
due to forward shocks. If the bulk Lorentz factor Gamma is decreasing in time,
the break ending the shallow phase can be explained, since at early times Gamma
is large, and we see only a fraction of the emitting area. Later, when Gamma
decreases, we see an increasing fraction of the emitting surface up to the time
when Gamma ~ 1/theta_j. This time ends the shallow phase of the X-ray light
curve. The origin of the late prompt emission can be the accretion of the
fall-back material, with an accretion rate dot M proportional to t^(-5/3). The
combination of this late prompt emission with the flux produced by the standard
forward shock can explain the great diversity of the optical and the X-ray
light curves.
