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The early X-ray afterglows of gamma-ray bursts (GRBs) are usually welldescribed by absorbed power laws. However, in some cases, additional thermalcomponents have been identified. The origin of this emission is debated, withproposed explanations including supernova shock breakout, emission from acocoon surrounding the jet, as well as emission from the jet itself. A largersample of detections is needed in order to place constraints on these differentmodels. Here we present a time-resolved spectral analysis of 74 GRBs observedby Swift XRT in a search for thermal components. We report six detections inour sample, and also confirm an additional three cases that were previouslyreported in the literature. The majority of these bursts have a narrow range ofblackbody radii around ~ 2 x 10^{12} cm, despite having a large range ofluminosities (L ~ 10^{47} - 10^{51} erg s^{-1}). This points to an originconnected to the progenitor stars, and we suggest that emission from a cocoonbreaking out from a thick wind may explain the observations. For two of thebursts in the sample, an explanation in terms of late prompt emission from thejet is instead more likely. We also find that these thermal components arepreferentially detected when the X-ray luminosity is low, which suggests thatthey may be hidden by bright afterglows in the majority of GRBs.

Thermal components in the early X-ray afterglows of GRBs: likely cocoon emission and constraints on the progenitors