Swift UVOT observations of X-ray flash 050406

We present Swift UVOT data on the optical afterglow of the X-ray flash of 2005 April 6 (XRF 050406) from 88 to ∼10⁵ s after the initial prompt γ-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 γ-ray 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 (fast-rise, exponential decay) profile with a duration of T₉₀ = 5.7 ± 0.2 s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption redward of 4000 Å 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 power-law decay with slope α = -0.75 ± 0.26(F_ν ∝ t ^α) and a maximum occurring in the first 200 s after the initial γ-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.