Dark Bursts in the Swift Era: The Palomar 60 inch-Swift Early Optical   Afterglow Catalog
  We present multi-color optical observations of long-duration gamma-ray bursts (GRBs) made over a three year period with the robotic Palomar 60 inch telescope (P60). Our sample consists of all 29 events discovered by Swift for which P60 began observations less than one hour after the burst trigger. We were able to recover 80% of the optical afterglows from this prompt sample, and we attribute this high efficiency to our red coverage. Like Melandri et al. (2008), we find that a significant fraction (~ 50%) of Swift events show a suppression of the optical flux with regards to the X-ray emission (so-called "dark" bursts). Our multi-color photometry demonstrates this is likely due in large part to extinction in the host galaxy. We argue that previous studies, by selecting only the brightest and best-sampled optical afterglows, have significantly underestimated the amount of dust present in typical GRB environments. 
S. B. Cenko, J. Kelemen, F. A. Harrison, D. B. Fox, S. R. Kulkarni, M.   M. Kasliwal, E. O. Ofek, A. Rau, A. Gal-Yam, D. A. ...