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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 P60began observations less than one hour after the burst trigger. We were able torecover 80% of the optical afterglows from this prompt sample, and we attributethis high efficiency to our red coverage. Like Melandri et al. (2008), we findthat a significant fraction (~ 50%) of Swift events show a suppression of theoptical flux with regards to the X-ray emission (so-called "dark" bursts). Ourmulti-color photometry demonstrates this is likely due in large part toextinction in the host galaxy. We argue that previous studies, by selectingonly the brightest and best-sampled optical afterglows, have significantlyunderestimated the amount of dust present in typical GRB environments.

Dark Bursts in the Swift Era: The Palomar 60 inch-Swift Early Optical Afterglow Catalog