Optical GRB Afterglows Detected with UVOT

Abstract

The automated response of the UltraViolet and Optical Telescope (UVOT) on Swift to new GRBs has several parameters, including exposure time, filter sequence and data mode, that can be adjusted to optimize the science return of early afterglow observations. After some initial changes, the response has remained stable since March 15, 2006. From then through August 10, 2007, UVOT observed 122 of the 130 GRBs detected with Swift's Burst Alert Telescope (BAT). UVOT typically takes an initial 100‐s exposure with the White filter (160–650 nm) starting 60–180 s after the trigger and then takes exposures with the other 6 filters. In its first finding chart exposure UVOT detected 39% of the 84 long (T90>2.0 s) GRBs that were not heavily reddened in the Milky Way (EB−V<0.5) and were observed within 500 seconds of the trigger. Another 4% were detected after including subsequent exposures. Afterglow magnitudes ranged from 12.8 to the sensitivity limit of ∼21. Only 1 of 11 short GRBs were detected, and its magnitude wa...