  It has been reported that some X-ray spectra of gamma-ray burst (GRB)
afterglows cannot be fitted by a simple power law. A blackbody component is
added to precisely fit the thermal feature in these spectra. Alternatively, we
propose that bremsstrahlung radiation can also be one possible mechanism to
explain the thermal component of the GRB X-ray afterglow. In particular, we
examine the X-ray afterglow of the ultra-long GRB 130925A in this paper. By our
calculation, we find that the X-ray thermal component observed by both
Swift-XRT and NuSTAR can be well explained by the bremsstrahlung radiation. Our
results indicate that the GRBs with the bremsstrahlung emission in the X-ray
afterglow could be born in a metal-rich and dusty environment.
