SWIFT observations of gamma-ray burst afterglows

We verify the photometric quality of the UVOT settling data using bright stars in the field of view. In the majority of cases, we find no problems with the settling exposure photometry, but in one case, we excise the first second of the exposure to mitigate a spacecraft attitude reconstruction issue, and in a second case, we exclude the first second of the exposure in which the UVOT photocathode voltage appears to be ramping up.

Of a sample of 23 afterglows which have peak V magnitudes <16, we find that all are detected in the settling exposures, when Swift arrives on target. For nine of the GRBs, the UVOT settling exposure took place before the conclusion of the prompt gamma-ray emission. Five of these GRBs have well-defined optical peaks after the settling exposures, with rises of >0.5mag in their optical light curves, and there is a marginal trend for these GRBs to have long T_90_.