摘要：Context. A thermal X-ray component is observed in the early afterglow of some gamma-ray bursts. Possible explanations include shockwave breakout, relativistic photosphere, or emission from cocoon. The difficulties of these models are discussed.
Aims. We propose an alternative model that attributes such a thermal component to the interaction of the gamma-ray burst outflow with a baryonic material near the gamma-ray burst source.
Methods. The analytic model is based on relativistic energy-momentum conservation and a diffusion model for photons. The kinematic and observational properties of the supernova ejecta after the interaction are derived. In particular, the temperature and the duration of the thermal emission are obtained.
Results. The model is applied to a prototypical GRB 090618 and other gamma-ray bursts associated with supernovae having thermal emission in the early afterglow. The mass of the baryonic material is found to be a few 10-4M⊙, indicating that this material can be a small fraction of the supernova ejecta.
