On the nature of the extremely fast optical rebrightening of the
  afterglow of GRB 081029
  Context. After the launch of the Swift satellite, the Gamma-Ray Burst (GRB)
optical light-curve smoothness paradigm has been questioned thanks to the
faster and better sampled optical follow-up, which has unveiled a very complex
behaviour. This complexity is triggering the interest of the whole GRB
community. The GROND multi-channel imager is used to study optical and
near-infrared (NIR) afterglows of GRBs with unprecedented optical and
near-infrared temporal and spectral resolution. The GRB 081029 has a very
prominent optical rebrightening event and is an outstanding example of the
application of the multi-channel imager to GRB afterglows. Aims. Here we
exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT
observations to study the nature of late-time rebrightenings that appear in the
optical-NIR light-curves of some GRB afterglows. Methods. We analyse the
optical and NIR observations obtained with the seven-channel Gamma-Ray burst
Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and
the X-ray data obtained with the XRT telescope on board the Swift observatory.
The multi-wavelength temporal and spectral evolution is discussed in the
framework of different physical models. Results. The extremely steep optical
and NIR rebrightening observed in GRB 081029 cannot be explained in the
framework of the standard forward shock afterglow model. The absence of a
contemporaneous X-ray rebrightening and the evidence of a strong spectral
evolution in the optical-NIR bands during the rise suggest two separate
components that dominate in the early and late-time lightcurves, respectively.
The steepness of the optical rise cannot be explained even in the framework of
the alternative scenarios proposed in the literature unless a late-time
activity of the central engine is assumed.
