Probing the circumstellar medium of GRB afterglows through absorption-line observations

A systematic search of Wolf-Rayet wind signatures, as represented by blue-shifted, high-velocity (|Deltav|=1000-5000 kms(-1)) C IV lambdalambda 1548, 1550 absorption doublet has yielded an estimate of 20% for the incidence of these C IV absorbers near the host galaxies of gamma-ray bursts (GRBs). This is consistent with what is observed near classical damped Lyalpha absorbers that have a comparable neutral hydrogen column density as the GRB host galaxies. A detailed ionization analysis of these absorbers, including the associated low-ionization species, shows that the majority in fact originate in foreground galaxies along the sightline, rather than in the vicinity of the GRB afterglows. Taking into account the enhanced afterglow radiation field, the lack of Wolf-Rayet signatures can be applied to constrain the C/He ratio and the density contrast of the winds in the vicinity of GRB progenitor stars.