According to the internal-external shocks model for γ-ray bursts
(GRBs), the GRB is produced by internal shocks within a relativistic flow while
the afterglow is produced by external shocks with the ISM. We explore the early
afterglow emission. For short GRBs the peak of the afterglow will be delayed,
typically, by few dozens of seconds after the burst. For long GRBs the early
afterglow emission will overlap the GRB signal. We calculate the expected
spectrum and the light curves of the early afterglow in the optical, X-ray and
γ-ray bands. These characteristics provide a way to discriminate
between late internal shocks emission (part of the GRB) and the early afterglow
signal. If such a delayed emission, with the characteristics of the early
afterglow, will be detected it can be used both to prove the internal shock
scenario as producing the GRB, as well as to measure the initial Lorentz factor
of the relativistic flow. The reverse shock, at its peak, contains energy which
is comparable to that of the GRB itself, but has a much lower temperature than
that of the forward shock so it radiates at considerably lower frequencies. The
reverse shock dominates the early optical emission, and an optical flash
brighter than 15th magnitude, is expected together with the forward shock peak
at x-rays or γ-rays. If this optical flash is not observed, strong
limitations can be put on the baryonic contents of the relativistic shell
deriving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure