We present a model for the early optical afterglow of GRB 021004. This burst
had one of the earliest detected optical afterglows, allowing a dense optical
sampling. The lightcurve was peculiar, since bright bumps were superimposed to
the regular power-law decay observed in many other events. We show that, given
their time scale and shape, the bumps are due to the interaction of the
fireball with moderate enhancements in the ambient medium density. The
enhancements have a density contrast of order 10, modifying only slightly the
dynamics of the fireball, which therefore surfs on them rather than colliding
onto them. A relativistic reverse shock does not develop. Since the interaction
takes place when the fireball is still hyper-relativistic it is not possible to
understand if the overdensities are localized in clumps or are spherically
symmetric around the GRB progenitor. The monotonic decrease of the contrast of
successive rebrightenings suggests however the presence of clumps in an average
uniform environment. Such an interpretation, complemented by the detection of
several high velocity absorption systems in the optical spectrum, strongly
suggests that GRB 021004 exploded within the remnant of a previous explosion.Comment: 5 pages, 2 postscript figures, A&A reference adde