GRB 081028 and its late time afterglow re-brightening

Swift captured for the first time a smoothly rising X-ray re-brightening of clear nonflaring 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 onset of the forward shock as the outflow decelerates as the origin of the observed re-brightening if an on-axis geometry is assumed. We investigate alternative origins and show that the observations are consistent with the off-axis jet predictions. The high on-axis energy budget implied by this interpretation suggests a different physical origin of the prompt and 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 Eiso measurements.