Circumburst Environments of Gamma-Ray Bursts

Abstract

I review current evidence on the surroundings of gamma‐ray bursts, in light of the fact that GRB 030329 was associated with the explosion of a massive star (SN 2003dh). Although interaction with either a free wind or the shocked wind from a massive star progenitor might be able to explain GRB observations, rather special circumstances are needed to enable this. In particular, a number of lines of evidence suggest a lower density than would be expected around a Wolf‐Rayet star for many bursts. The recent observation of absorption lines in GRB spectra may indicate radiatively accelerated clumps in the circumburst environment, but an outflow from the host galaxy cannot be ruled out.