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We examine the the emission from optically bright gamma-ray burst (GRB)afterglows as the Ultraviolet and Optical Telescope (UVOT) on the Neil GehrelsSwift Observatory first begins observing, following the slew to target the GRB,while the pointing of the Swift satellite is still settling. We verify thephotometric quality of the UVOT settling data using bright stars in the fieldof view. In the majority of cases we find no problems with the settlingexposure photometry, but in one case we excise the first second of the exposureto mitigate a spacecraft attitude reconstruction issue, and in a second case weexclude the first second of the exposure in which the UVOT photocathode voltageappears to be ramping up. Of a sample of 23 afterglows which have peak Vmagnitudes <16, we find that all are detected in the settling exposures, whenSwift arrives on target. For 9 of the GRBs the UVOT settling exposure tookplace before the conclusion of the prompt gamma-ray emission. Five of theseGRBs have well defined optical peaks after the settling exposures, with risesof >0.5 mag in their optical lightcurves, and there is a marginal trend forthese GRBs to have long T90. Such a trend is expected for thick-shellafterglows, but the temporal indices of the optical rises and the timing of theoptical peaks appear to rule out thick shells.

A study of gamma-ray burst afterglows as they first come into view of the Swift Ultraviolet and Optical Telescope