Photospheres, Comptonization and X-ray lines in gamma-ray bursts

Steep X-ray spectral slopes, X-ray excesses and preferred spectral energy breaks in the 0.1–0.3 MeV range are discussed as the possible consequences of the photospheric component of the GRB relativistic outflow, and of pair breakdown in internal shocks leading to Comptonization on semi-relativistic electrons or MHD waves. We also discuss the X-ray and UV spectra of GRB afterglows occurring in a dense environment characteristic of massive stellar progenitors, including their ability to produce detectable Fe or other metallic line features.