Thermal components in the early X-ray afterglow of GRBs

The possible presence of thermal components in the early X-ray afterglows of gamma-ray bursts is investigated. We discuss both the presence of a thermal continuum and, in particular, of collisional X-ray emission lines. We compute the predicted luminosity by a thin plasma for a range of metallicities for the continuum and the K_alpha lines of the elements Mg, Si, S, Ar, Ca and Fe. We show that light travel effects are dominant in the determination of the thermal continuum and line luminosities, and derive the relevant equations. We conclude that thermal lines and continua are unlikely to dominate the early afterglow of GRBs, unless the explosion site is surrounded by a very massive and extremely clumped shell of material. Such conditions are difficult to envisage in the close environment of GRB progenitor, unless they are excited by some strong precursor activity, like in the Supranova scenario.