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

TL;DR

This study presents early multi-color optical observations of 29 Swift-detected long-duration gamma-ray bursts using the Palomar 60 inch telescope, revealing high detection efficiency and significant host galaxy extinction in dark bursts.

Contribution

It provides the first comprehensive early optical afterglow catalog for Swift GRBs and highlights the impact of host galaxy dust on optical flux suppression.

Findings

80% optical afterglow recovery rate due to red coverage

Approximately 50% of Swift GRBs are optically dark

Host galaxy extinction accounts for optical flux suppression

Abstract

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.