MG15 - Talk detail

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The early X-ray afterglows of GRBs are usually well-described by a simple absorbed power-law. However, in 2006, in the afterglow of GRB 060218, it was shown that an additional component in the form of blackbody was required to explain the spectra. Since then there has been a dozen other reports of thermal components in the early X-ray afterglows of GRBs. The origin of this emission is still unclear, with proposed explanations including supernova shock breakout, the cocoon surrounding the jet or emission from the jet itself. In this work we present a systematic study of 74 GRBs with known redshifts observed by Swift XRT in a search for thermal components in the early X-ray afterglows. We report six detections in our sample, and also confirm an additional three cases that were previously reported in the literature. We explore their common properties and find that the majority of these bursts have a narrow span of radii, while at the same time having a wide spread in luminosities. From these results we infer that a likely explanation for the thermal emission is the cocoon breaking out from a thick wind that surrounds the progenitor star. For two of the bursts an explanation in terms of late prompt emission from the jet is instead more likely.

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