ar X iv : a st ro - p h / 06 01 18 2 v 2 2 1 M ar 2 00 6 Swift - UVOT Observations of the 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 s to ∼ 10 5 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-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 T 90 = 5.7 s ± 0.2 s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption red-ward of 4000Å 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.