Swift Uvot Observations of X-ray Flash 050406

We present SwiftUVOT data on the optical afterglow of the X-ray flash of 2005 April 6 (XRF 050406) from 88 to 10 s after the initial prompt -ray emission. Our observations in the V, B, and U bands are the earliest that have been taken of an XRF optical counterpart. Combining the early-time optical temporal and spectral properties with -ray and simultaneous X-ray data taken with the BATand XRT telescopes on board Swift, we are able to constrain possible origins of the XRF. The prompt emission had a FRED (fast-rise, exponential decay) profile with a duration of T90 1⁄4 5:7 0:2 s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption redward of 40008 in the UV/optical spectrum provides a firm upper limit of z 3:1 on the redshift, thus excluding a high redshift as the sole reason for the soft spectrum. The optical light curve is consistent with a powerlaw decay with slope 1⁄4 0:75 0:26(F / t ) and a maximum occurring in the first 200 s after the initial -ray emission. The softness of the prompt emission is well described by an off-axis structured jet model, which is able to account for the early peak flux and shallow decay observed in the optical and X-ray bands. Subject headings: gamma rays: bursts — X-rays: individual (XRF 050406)