Astrophys. J., 643, 276-283 (2006/May-3)
Swift UVOT observations of X-ray flash 050406.
SCHADY P., MASON K.O., OSBORNE J.P., PAGE M.J., ROMING P.W.A., STILL M., ZHANG B., BLUSTIN A.J., BOYD P., CUCCHIARA A., GEHRELS N., GRONWALL C., DE PASQUALE M., HOLLAND S.T., MARSHALL F.E., McGOWAN K.E. and NOUSEK J.A.
Abstract (from CDS):
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(Fν∝tα) 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.
Gamma Rays: Bursts - X-rays: individual (XRF 050406)
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