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Thermal radiation, peaking in soft X-rays, has now been detected in a handfulof GRB afterglows and has to date been interpreted as shock break-out of theGRB's progenitor star. We present a search for thermal emission in the earlyX-ray afterglows of a sample of Swift bursts selected by their brightness inX-rays at early times. We identify a clear thermal component in eight GRBs andtrack the evolution. We show that at least some of the emission must come fromhighly relativistic material since two show an apparent super-luminal expansionof the thermal component. Furthermore we determine very large luminosities andhigh temperatures for many of the components-too high to originate in a SNshock break-out. Instead we suggest that the component may be modelled as latephotospheric emission from the jet, linking it to the apparently thermalcomponent observed in the prompt emission of some GRBs at gamma-ray and hardX-ray energies. By comparing the parameters from the prompt emission and theearly afterglow emission we find that the results are compatible with theinterpretation that we are observing the prompt quasi-thermal emissioncomponent in soft X-rays at a later point in its evolution.

Thermal emission in the early X-ray afterglows of GRBs: following the prompt phase to the late times