2001ApJ...562..664S

According to recent models, γ-ray bursts apparently explode in a wide variety of ambient densities ranging from ∼10^–3 to 30 cm^–3 or more. The lowest density environments seem, at first sight, to be incompatible with bursts in or near molecular clouds or with dense stellar winds and hence with the association of γ-ray bursts with massive stars. We argue that low ambient density regions naturally exist in areas of active star formation as the interiors of superbubbles. The evolution of the interior bubble density as a function of time for different assumptions about the evaporative or hydrodynamical mass loading of the bubble interior is discussed. We present a number of reasons why there should exist a large range of inferred afterglow ambient densities whether γ-ray bursts arise in massive stars or some version of compact star coalescence. We predict that many γ-ray bursts will be identified with X-ray bright regions of galaxies, corresponding to superbubbles, rather than with blue localized regions of star formation. The lack of evidence for winds may imply low wind densities and hence low mass-loss rates combined with high velocities for any massive star progenitor. The problem would be avoided in a binary neutron star model.