The Diversity of Gamma‐Ray Burst Afterglows and the Surroundings of Massive Stars

Abstract

The finding of a Type Ic supernova connected with GRB 030329 showed a massive star origin for this burst, supporting evidence for this association in previous bursts with light-curve bumps at the appropriate time for a supernova. Here we explore the possibility that all long bursts have massive star progenitors, interacting with either the freely expanding wind of the progenitor or the shocked wind. We present models for the afterglows of GRB 020405 and GRB 021211, which are a challenge to wind interaction models. Considering sources for which wind interaction models are acceptable, a range of wind densities is required, from values typical of Galactic Wolf-Rayet stars to values ~102 times smaller. The reason for the low densities is unclear, but it may involve low progenitor masses and/or low metallicities. If mass is a factor, a low-density event should be associated with a low-mass supernova. The interpretation of bursts apparently interacting with constant density media as interaction with a shocked wind requires both a range of mass-loss densities and a range of external pressures. The highest pressures, p/k 108 cm-3 K, may be due to an extreme starburst environment, which would imply that the burst is superposed on an active star-forming region. Although the range of observed events can be accommodated by the shocked-wind theory, special circumstances are necessary to bring this about. Finally, we consider the high-velocity, high-ionization absorption features observed in some afterglow spectra. If the features are circumstellar, the presence of the burst in a starburst region may be important for the formation of clumps near the burst.