We present Swift -UVOT data on the optical afterglow of the X-ray flash of 2005 April 6 ( XRF 050406 ) from 88 s to \sim 10 ^ { 5 } s after the initial prompt \gamma -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 \gamma - and simultaneous X-ray data taken with the BAT and XRT telescopes on-board Swift , we are able to constrain possible origins of the XRF .
The prompt emission had a FRED profile ( fast-rise , exponential decay ) with a duration of \mathrm { T } _ { 90 } = 5.7 ~ { } \mathrm { s } \pm 0.2 ~ { } \mathrm { s } , putting it at the short end of the long-burst duration distribution .
The absence of photoelectric absorption red-ward of 4000 Å in the UV/optical spectrum provides a firm upper limit of \mathrm { z } \leq 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 power-law decay with slope \alpha = -0.75 \pm 0.26 ~ { } ( F _ { \nu } \propto t ^ { \alpha } ) , and a maximum occurring in the first 200 s after the initial \gamma -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 .