The Optically Dark Gamma Ray Burst Population

The Swift satellite has now detected more than 500 long-duration gamma ray bursts (GRBs), but statistical analysis remains challenging because the sub-sample with redshifts is relatively small and potentially biased. In this work we construct a more homogenous sample by imposing selection criteria designed to remove bursts which were not easily observable by large ground-based telescopes. The resulting fraction is more complete in terms of redshifts, with ~89% of bursts in our sample having spectroscopically or photometrically constrained redshifts as opposed to ~25% of the full Swift sample. Based on our sample, we find the fraction of Swift bursts occurring at redshifts of z > 6 to be in the range 2 – 23%. We use this sample to constrain the fraction of Swift bursts which are ‘dark’, i.e. those for which the optical emission seems to be suppressed relative to the X-ray. Defining a burst to be dark by the criteria of Jakobsson et al. (2004), we find a dark burst fraction in the range 16 – 58%. Of these, we find the fraction of dark bursts occurring at z > 6 to be in the range 4.5 – 28%, and thus the fraction of dark bursts occurring at redshifts of z < 6 to be ~72%. From this we conclude that only a small fraction of dark bursts are caused by suppression of the optical afterglow due to an extreme redshift, and that the dominant cause of dark GRBs is dust extinction. Given that we have shown a substantial fraction of Swift GRBs are dark, and a substantial fraction of these are due to dust extinction, we conclude that a significant fraction of GRBs occur in dusty environments, despite a preference for low metallicity environments. In agreement with recent authors, we believe that most dark GRBs are caused by moderate levels of dust at moderate redshifts (AV = 0.5 – 2.0, z = 1 – 3), and show from redshift distributions derived from our sample that the largest fraction of Swift GRBs (and dark GRBs) occur at these redshifts, coincident with the vigorous epoch of star formation believed to have taken place in dusty environments at these redshifts (Hopkins and Beacom 2006).