Did we observe the supernova shock breakout in GRB 060218?

The early optical data of GRB 060218 (the first 10(5) s after the trigger) have been interpreted as blackbody emission associated with the shock breakout of the associated supernova. If so, it is possible to infer lower limits to the bolometric luminosity and energetics of such a blackbody component. These limits, which are independent of the emissivity time dependence, are tighter for the very early data and correspond to energetics similar to 10(51) erg, too large to be produced by the breakout of a supernova shock. A further problem with the above interpretation concerns the luminosity of the observed X-ray blackbody component. It should be produced, in the shock breakout interpretation, as blackbody emission of approximately constant temperature from a surface area only slowly increasing with time. Although it has been suggested that, assuming anisotropy, the long duration of the X-ray blackbody component is consistent with a supernova shock breakout, the nearly constant size of the emitting surface requires some fine tuning. These difficulties support an alternative interpretation, according to which the emission follows the late dissipation of the fireball bulk kinetic energy. This in turn requires a small value of the bulk Lorentz factor.