Swift-UVOT Observations of the 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 88s to \sim 10^5s 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 T_90 = 5.7\pm 0.2s, putting it at the short end of the long-burst duration distribution. The absence of photoelectric absorption red-ward of 4000 \AA in the UV/optical spectrum provides a firm upper limit of 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 200s 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.
Keywords
Cite
@article{arxiv.astro-ph/0601182,
title = {Swift-UVOT Observations of the X-Ray Flash 050406},
author = {P. Schady and K. O. Mason and J. P. Osborne and M. J. Page and P. W. A. Roming and M. Still and B. Zhang and A. J. Blustin and P. Boyd and A. Cucchiara and N. Gehrels and C. Gronwall and M. De Pasquale and S. T. Holland and F. E. Marshall and K. E. McGowan and J. A. Nousek},
journal= {arXiv preprint arXiv:astro-ph/0601182},
year = {2009}
}
Comments
14 pages, 4 figures, accepted for publication in ApJ; typos corrected and upper limits in table 1 changed from background subtracted count rate in extraction region to the error associated with this