High column densities and low extinctions of gamma-ray bursts: evidence for hypernovae and dust destruction

We analyze a complete sample of gamma-ray burst afterglows and find X-ray evidence for high column densities of gas around them. The column densities are in the range 1022-1023 cm-2, which is right around the average column density of Galactic giant molecular clouds. We also estimate the cloud sizes to be 10-30 pc, implying masses >~105 Msolar. This strongly suggests that gamma-ray bursts lie within star-forming regions and therefore argues against neutron star mergers and for collapses of massive stars as their sources. The optical extinctions, however, are 10-100 times smaller than expected from the high column densities. This confirms theoretical findings that the early hard radiation from gamma-ray bursts and their afterglows can destroy the dust in their environment, thus carving a path for the afterglow light out of the molecular cloud. Because of the self-created low extinction and location in star-forming regions, we expect gamma-ray bursts to provide a relatively unbiased sample of high-redshift star formation. Thus, they may help resolve what is the typical environment of high-redshift star formation