Grb060218: a Relativistic Supernova Shock Breakout

We show that the prompt and afterglow X-ray emission of GRB060218, as well as its early (t 1 d) optical-UV emission, can be explained by a model in which a radiation-mediated shock propagates through a compact progenitor star into a dense wind. The prompt thermal X-ray emission is produced in this model as the mildly relativistic shock, β ≈ 0.85 carrying few ×10 49 erg, reaches the wind (Thomson) photosphere, where the post-shock thermal radiation is released and the shock becomes collisionless. Adopting this interpretation of the thermal X-ray emission, a subsequent X-ray afterglow is predicted, due to synchrotron emission and inverse-Compton scattering of SN UV photons by electrons accelerated in the collisionless shock. Early optical-UV emission is also predicted, due to the cooling of the outer δM ∼ 10 −3 M ⊙ envelope of the star, which was heated to high temperature during shock passage. The observed X-ray afterglow and the early optical-UV emission are both consistent with those expected in this model. Detailed analysis of the early optical-UV emission may provide detailed constraints on the density distribution near the stellar surface. In our previous paper (Campana et al. 2006) the points mentioned in the abstract were outlined only briefly, using order-of-magnitude arguments and with very little explanation, due to space limitations. Here we present a more detailed explanation and analysis of the prompt thermal X-ray emission and X-ray afterglow, as well as a calculation of the early optical-UV emission, and show that some claims made in recent publications (Ghisellini et al. 2006; Fan et al. 2006; Li 2006), according to which the observations are inconsistent with the massive wind interpretation, are not valid. GRB060218 was unique mainly in two respects: it showed a strong thermal X-ray emission accompanying the prompt non-thermal emission, and a strong optical-UV emission at early, t 1 d, time. We show here that these features, as well as the X-ray afterglow, can all be explained by a model in which a radiation mediated shock propagates through a compact progenitor star into a massive wind. We have shown in a separate paper (Wang et al. 2006) that the prompt non-thermal X-ray emission can also be explained by this model. As detailed below, the prompt thermal X-ray emission can be explained as shock breakout at a radius of ∼ 5 × 10 12 cm, which requires a Thomson optical depth (of the plasma ahead of the …