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We call "prompt" emission of Gamma Ray Bursts (GRBs) the erratic and violentphase of hard X-ray and soft gamma-ray emission, usually lasting for tens ofseconds 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", seenespecially in the X-ray band, characterised by a "steep-flat-steep" lightcurve, 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-rayemission with a unique component. Two different mechanisms seem to be required.One can well be the standard emission from the forward shock of the fireballrunning through the interstellar medium. The second one is more elusive, and tocharacterise its properties we have studied those GRBs with well sampled dataand known redshift, fitting at the same time the optical and the X-ray lightcurves by means of a composite model, i.e. the sum of the standard forwardshock emission and a phenomenological additional component with a minimum offree parameters. Some interesting findings have emerged, pointing to a longactivity of the central engine powered by fallback material.

Prolonged activity of the central engine of Gamma Ray Bursts