A large fraction of gamma-ray bursts (GRBs) shows a plateau phase during the
X-ray afterglow emission, whose physical origin is still debated. In this work
we define a sample of 30 GRBs with simultaneous X-ray and optical data during
and after the plateau phase. Through a time-resolved spectral analysis of the
X-ray plateaus, we test the consistency of the unabsorbed optical fluxes with
those obtained via X-ray-to-optical spectral extrapolation by assuming a
synchrotron spectrum. Combining X-ray with optical data, we find that 63%
(19/30) GRBs are compatible with a single synchrotron spectrum thus suggesting
that both the optical and X-ray radiations are produced from a single emitting
region. For these GRBs we derive the temporal evolution of the break frequency
and we compare it with the expectations predicted by several models. For 11/30
GRBs the optical emission is above the predicted range of values extrapolated
from the X-rays in at least one temporal bin of the light curve. These GRBs may
not be explained with a single zone emission, indicating the necessity of
invoking two cooperating processes in order to explain the broad band spectral
behaviour during X-ray plateaus. We discuss our findings in the framework of
different scenarios invoked to explain the plateau feature, including the
energy injection from a spinning-down magnetar and the high latitude emission
from a structured jet.Comment: submitted to A&