  We propose an alternative explanation for the post-breakout emission of SN
2008D associated with the X-ray transient 080109. Observations of this object
show a very small contrast of 0.35 dex between the light-curve minimum
occurring soon after the breakout, and the main luminosity peak that is due to
radioactive heating of the ejecta. Hydrodynamical models show that the cooling
of a shocked Wolf-Rayet star leads to a much greater difference (> 0.9 dex).
Our proposed scenario is that of a jet produced during the explosion which
deposits 56Ni-rich material in the outer layers of the ejecta. The presence of
high-velocity radioactive material allows us to reproduce the complete
luminosity evolution of the object. Without outer 56Ni it could be possible to
reproduce the early emission purely from cooling of the shocked envelope by
assuming a larger progenitor than a Wolf-Rayet star, but that would require an
initial density structure significantly different from what is predicted by
stellar evolution models. Analytic models of the cooling phase have been
proposed reproduce the early emission of SN 2008D with an extended progenitor.
However, we found that the models are valid only until 1.5 days after the
explosion where only two data of SN 2008D are available. We also discuss the
possibility of the interaction of the ejecta with a binary companion, based on
published analytic expressions. However, the binary separation required to fit
the early emission should be < 3 Rsun which is too small for a system
containing two massive stars.
