Swift UVOT observations of X-ray flash 050406

Abstract

We present Swift UVOT data on the optical afterglow of the X-ray flash of 2005 April 6 ( XRF 050406) from 88 to similar to 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-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 T-90 = 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 (A) over circle 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 alpha = 0.75 +/- 0.26( F-v proportional to 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.