  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.
