Puzzled by GRB 060218

Abstract

Abstract We study the optical–UV/X-ray spectral energy distribution of GRB 060218 during the prompt and what seems to be the afterglow phases. The results are puzzling, since if the optical–UV and the X-ray emission belong to a single blackbody then its luminosity is too large, and this blackbody cannot be interpreted as the signature of the shock breakout of the supernova. There are also serious problems in associating the emission expected from the supernova shock breakout with either the optical–UV or the X-ray emission. In the former case we derive too small ejecta velocities; in the latter case, in contrast, the required velocity is too large, corresponding to the large radius of a blackbody required to peak close to the UV band. We then present what we think is the most conservative alternative explanation, namely a synchrotron spectrum, self-absorbed in the optical–UV and extending up to the X-ray band, where we observe the emission of the most energetic electrons, which are responsible for the exponential roll-over of the spectrum. The obtained fit can explain the entire spectrum except the blackbody observed in the X-rays, which must be a separate component. The puzzling feature of this interpretation is that the same model is required to explain the spectrum also at later times, up to 105 s, because the optical–UV emission remains constant in shape and also (approximately) in normalization. In this case the observed X-ray flux is produced by self-Compton emission. Thus the prompt emission phase should last for ∼105 s or more. Finally, we show that the blackbody observed in X-rays, up to 7000 s, can be photospheric emission from the cocoon or stellar material, energized by the gamma-ray burst jet at radii comparable to the stellar radius (i.e. 1010–1011 cm), not very far from where this material becomes transparent (e.g. 1012 cm).