  Four years after the launch the Swift satellite the nature of the GRBs
broadband afterglow behaviour is still an open issue. The standard external
shock fireball model cannot easily explain the combined temporal and spectral
properties of Optical to X-ray afterglows. We analysed the rest frame
de-absorbed and K-corrected Optical and X-ray light curves of a sample of 33
GRBs with known redshift and optical extinction at the host frame. We modelled
their broadband behaviour as the sum of the standard forward shock emission due
to the interaction of a fireball with the circum-burst medium and an additional
component. This description provides a good agreement with the observed light
curves despite their complexity and diversity and can also account for the lack
of achromatic late times jet breaks and the presence of chromatic breaks in
several GRBs lightcurves. In order to test the predictions of such modelling we
analysed the X-ray time resolved spectra searching for possible spectral breaks
within the observed XRT energy band, finding 7 GRBs showing such a break. The
Optical to X-Ray SED evolution of these GRBs are consistent with what expected
by our interpretation.
