  Swift captured for the first time a smoothly rising X-ray re-brightening of
clear non-flaring origin after the steep decay in a long gamma-ray burst (GRB):
GRB081028. A rising phase is likely present in all GRBs but is usually hidden
by the prompt tail emission and constitutes the first manifestation of what is
later to give rise to the shallow decay phase. Contemporaneous optical
observations reveal a rapid evolution of the injection frequency of a fast
cooling synchrotron spectrum through the optical band, which disfavours the
afterglow onset (start of the forward shock emission along our line of sight
when the outflow is decelerated) as the origin of the observed re-brightening.
We investigate alternative scenarios and find that the observations are
consistent with the predictions for a narrow jet viewed off-axis. The high
on-axis energy budget implied by this interpretation suggests different
physical origins of the prompt and (late) afterglow emission. Strong spectral
softening takes place from the prompt to the steep decay phase: we track the
evolution of the spectral peak energy from the gamma-rays to the X-rays and
highlight the problems of the high latitude and adiabatic cooling
interpretations. Notably, a softening of both the high and low spectral slopes
with time is also observed. We discuss the low on-axis radiative efficiency of
GRB081028 comparing its properties against a sample of Swift long GRBs with
secure E_gamma,iso measurements.
